TOKYO—Tokyo reported its lowest daily level of new coronavirus cases in more than three weeks, a fresh sign that Japan may be succeeding in suppressing the outbreak without compulsory social distancing and with limited testing.

Unlike many Western countries, Japan hasn’t imposed a lockdown backed with fines or other penalties. Instead, Prime Minister Shinzo Abe has asked people to reduce their contact with others by 80% and encouraged businesses to reduce hours of operation or close. Restaurants may open but are asked to close by 8 p.m.

Japan has also shunned widespread testing for the virus, instead focusing on targeting clusters of infection when they appear in an effort to snuff out any further spread.

On Monday, Tokyo reported 39 new cases of infection in the capital, the lowest total since March 30. New cases are often lower on Mondays because they reflect test results from Sunday, when many hospitals are closed, but the latest figure was less than half that of every other Monday in April.

The downtrend in new cases is also reflected in national data. Across Japan, the rate of increase of coronavirus infections fell to 1.5% on Sunday from 12% a day after Mr. Abe asked people in major cities to reduce activity under a state of emergency on April 7.