The Tokyo Immigration Bureau on Wednesday detained 13 Afghan and Pakistani nationals in Tokyo and Chiba Prefecture on suspicion of violating the Immigration Control Law.
Of the 13, 11 are Afghan men and two are Pakistani men.
Bureau officials said the 13 are not believed to be connected to the Sept 11. terrorist attacks in the United States.
The officials quoted the Afghans as saying they illegally entered Japan between February 2000 and September this year by such means as presenting false passports and arriving by cargo ship.
The 13 were all working illegally at places including auto dismantling plants, the officials said.
"The 13 were detained for illegally staying (in Japan)," Chief Cabinet Secretary Yasuo Fukuda said. "From what I understand, they have nothing to do with the terrorist attacks."