Saitama Prefectural Police arrested a woman on suspicion of violating the immigration law but later released her after discovering she is a Japanese national, police officials said Monday.
Police concluded the 28-year-old unemployed woman was not Japanese because she looked like a foreigner of Asian descent and was carrying an envelope with Portuguese writing on it, the officials said.
The woman was questioned by an officer at around 7:40 p.m. Saturday in Kawaguchi. She told the officer she was Japanese but refused to answer further questions, the officials said.
The woman's family said she is not good at speaking with strangers.
Failing to confirm the woman's identity, police obtained a warrant and arrested her at around 5 a.m. Sunday.
When the woman's 58-year-old mother was shown a photo of her daughter, she told police she "did not know" her. The woman was released at 7:20 p.m. Sunday after the mother finally said it was her daughter.
"We have caused a lot of trouble. From now on, we will do our best not to let this happen again by issuing thorough instructions," said Satoshi Kanagawa, chief of the Kawaguchi Police Station.
The Saitama force has been involved in a series of erroneous arrests in recent years, including in May, when they arrested a man based on a malicious claim he had stolen a wallet.