Fujitsu Ltd. said the computer servers it uses for online local public services came under attack between noon Wednesday and early Thursday.
Public online services were briefly incapacitated but no information was leaked after repeated, numerous access attempts through about 30 Internet protocol addresses overwhelmed systems in nine prefectures, according to Fujitsu.
In the city of Fukuoka, for example, online services were interrupted intermittently for a total of six hours and 40 minutes starting Wednesday afternoon.
The nine prefectures were Fukushima, Chiba, Nagano, Shizuoka, Mie, Shimane, Yamaguchi, Fukuoka and Kagoshima.
Fujitsu said its servers are used by around 200 local authorities in 10 prefectures.
The incident came on the heels of a series of attacks on the computer networks of the Diet and major firms, including Mitsubishi Heavy Industries, Japan's largest defense contractor.
Information on MHI's military aircraft may have been read by outsiders during the attack.