A group of 21 officials and trainees from the National Institute of Infectious Diseases dined together in Tokyo last month, while the capital was under the coronavirus state of emergency.
The health ministry says the gathering took place on March 18 at the institute's compound in Tokyo's Shinjuku Ward. It says the trainees had completed an epidemiological survey course. The ministry added that alcohol was also served at the event, which lasted for about 15 minutes.
The participants reportedly sat in groups of up to four people and were wearing masks.
The ministry said anti-infection measures were in place, indicating that it will not punish those involved.
But it instructed the institute not to hold such gatherings again because they could be misunderstood by the public.