* asymptomatic stage 1 patients * patients with spo2 less than 95% without oxygen therapy * patients who needs oxygen supplements * patients with concomitant bacterial or fungal infection (confirmed by culture) prior to initiation of study * patients with congestive heart failure * patients with severe hepatic impairment (\>grade 3: alt \>10 times of upper normal limit) * impaired kidney function (creatinine clearance according to cockcroft-gault formula less than 30 ml/min) at the time of screening. * malabsorption syndrome or other clinically significant gastrointestinal disease that may affect absorption of the study drug (non-correctable vomiting, diarrhea, ulcerative colitis, and others). * pregnant or nursing women or women planning pregnancy. * female patients who cannot consent to contraceptive use of oral contraceptives, mechanical contraceptives such as intrauterine devices or barrier devices (pessaries, condoms), or a combination of these devices from the start of favipiravir administration to 7 days after the end of favipiravir administration * male patients whose partner cannot agree to use the contraception method described in (9) * patients with a history of gout or on treatment for gout or hyperuricemia * patients receiving immunosuppressants * patients who received interferon or drugs with reported antiviral activity against covid-19 (hydroxychloroquine sulfate, chloroquine phosphate, lopinavir-ritonavir combination) within 7 days of illness. * patients in whom this episode of infection is a recurrence or reinfection of covid-19 infection * patients who have previously received favipiravir * patients who are not able to provide written consent by themselves * other patients judged ineligible by the principal investigator or sub-investigator

* asymptomatic stage 1 patients * patients with spo2 less than 95% without oxygen therapy * patients who needs oxygen supplements * patients with concomitant bacterial or fungal infection (confirmed by culture) prior to initiation of study * patients with congestive heart failure * patients with severe hepatic impairment (\>grade 3: alt \>10 times of upper normal limit) * impaired kidney function (creatinine clearance according to cockcroft-gault formula less than 30 ml/min) at the time of screening. * malabsorption syndrome or other clinically significant gastrointestinal disease that may affect absorption of the study drug (non-correctable vomiting, diarrhea, ulcerative colitis, and others). * pregnant or nursing women or women planning pregnancy. * female patients who cannot consent to contraceptive use of oral contraceptives, mechanical contraceptives such as intrauterine devices or barrier devices (pessaries, condoms), or a combination of these devices from the start of favipiravir administration to 7 days after the end of favipiravir administration * male patients whose partner cannot agree to use the contraception method described in (9) * patients with a history of gout or on treatment for gout or hyperuricemia * patients receiving immunosuppressants * patients who received interferon or drugs with reported antiviral activity against covid-19 (hydroxychloroquine sulfate, chloroquine phosphate, lopinavir-ritonavir combination) within 7 days of illness. * patients in whom this episode of infection is a recurrence or reinfection of covid-19 infection * patients who have previously received favipiravir * patients who are not able to provide written consent by themselves * other patients judged ineligible by the principal investigator or sub-investigator

- asymptomatic stage 1 patients - patients with spo2 less than 95% without oxygen therapy - patients who needs oxygen supplements - patients with concomitant bacterial or fungal infection (confirmed by culture) prior to initiation of study - patients with congestive heart failure - patients with severe hepatic impairment (>grade 3: alt >10 times of upper normal limit) - impaired kidney function (creatinine clearance according to cockcroft-gault formula less than 30 ml/min) at the time of screening. - malabsorption syndrome or other clinically significant gastrointestinal disease that may affect absorption of the study drug (non-correctable vomiting, diarrhea, ulcerative colitis, and others). - pregnant or nursing women or women planning pregnancy. - female patients who cannot consent to contraceptive use of oral contraceptives, mechanical contraceptives such as intrauterine devices or barrier devices (pessaries, condoms), or a combination of these devices from the start of favipiravir administration to 7 days after the end of favipiravir administration - male patients whose partner cannot agree to use the contraception method described in (9) - patients with a history of gout or on treatment for gout or hyperuricemia - patients receiving immunosuppressants - patients who received interferon or drugs with reported antiviral activity against covid-19 (hydroxychloroquine sulfate, chloroquine phosphate, lopinavir-ritonavir combination) within 7 days of illness. - patients in whom this episode of infection is a recurrence or reinfection of covid-19 infection - patients who have previously received favipiravir - patients who are not able to provide written consent by themselves - other patients judged ineligible by the principal investigator or sub-investigator

- asymptomatic stage 1 patients - patients with spo2 less than 95% without oxygen therapy - patients who needs oxygen supplements - patients with concomitant bacterial or fungal infection (confirmed by culture) prior to initiation of study - patients with congestive heart failure - patients with severe hepatic impairment (>grade 3: alt >10 times of upper normal limit) - impaired kidney function (creatinine clearance according to cockcroft-gault formula less than 30 ml/min) at the time of screening. - malabsorption syndrome or other clinically significant gastrointestinal disease that may affect absorption of the study drug (non-correctable vomiting, diarrhea, ulcerative colitis, and others). - pregnant or nursing women or women planning pregnancy. - female patients who cannot consent to contraceptive use of oral contraceptives, mechanical contraceptives such as intrauterine devices or barrier devices (pessaries, condoms), or a combination of these devices from the start of favipiravir administration to 7 days after the end of favipiravir administration - male patients whose partner cannot agree to use the contraception method described in (9) - patients with a history of gout or on treatment for gout or hyperuricemia - patients receiving immunosuppressants - patients who received interferon or drugs with reported antiviral activity against covid-19 (hydroxychloroquine sulfate, chloroquine phosphate, lopinavir-ritonavir combination) within 7 days of illness. - patients in whom this episode of infection is a recurrence or reinfection of covid-19 infection - patients who have previously received favipiravir - patients who are not able to provide written consent by themselves - other patients judged ineligible by the principal investigator or sub-investigator