Officials in Saitama Prefecture, near Tokyo, have culled about 800 pigs after swine fever was detected at a hog farm.
The prefectural government started putting down pigs at the farm in Chichibu City late on Friday after the virus was detected earlier in the day.
About 150 people took part in the work through early Sunday.
Prefectural officials say they will bury the culled animals and decontaminate the farm to prevent the spread of the virus. They expect to complete the work on Monday.
The officials will also disinfect vehicles that come in and out of farms within 10 kilometers of the stricken farm until mid-October.
They will conduct on-site inspections in the event of any irregularity, such as pig deaths.
Swine fever was confirmed in 2018 in Gifu Prefecture, central Japan, for the first time in 26 years in the country.
This is the first case in the Kanto region around Tokyo, which is home to nearly a quarter of Japan's pigs.
Saitama prefectural officials are working to determine the source of the infection.