Police and immigration officers arrested or temporarily detained 273 illegal immigrants in Chiba Prefecture during an intensive investigation in September, officials said Tuesday.
The number brings the total for Chiba from January to September to 633, surpassing last year's tally of 429, they said.
They consisted of 454 men and 179 women. By country, 175 are from the Philippines, 116 from Thailand, 75 from Bangladesh and 68 from China. About 80 percent were detained for illegally staying in Japan, while the others were held for illegal entry, the officials said.
Many were working as construction workers or hostesses.
The sweep came after the Justice Ministry's Immigration Bureau dispatched specialized immigrant-tracking officers to its branches in Chiba and Saitama prefectures in April. The bureau believes about 70 percent of illegal immigrants in Japan live in Tokyo and its vicinity.
The government estimates there are about 250,000 foreigners illegally residing in the country.