A lawmaker from Japan's governing party has been arrested on suspicion of receiving 3.7m yen ($34,000) in bribes from a gambling operator.
As a member of Prime Minister Shinzo Abe's cabinet until October, Tsukasa Akimoto oversaw the government's plan to introduce casinos.
The 48-year-old denies wrongdoing and says he never extended any favours.
But correspondents say his arrest could complicate Mr Abe's controversial policy on casinos.
Mr Akimoto is accused of receiving money from three employees of an unnamed gambling operator seeking help for a casino bid, prosecutors say.
The three employees were also detained on Wednesday, prosecutors said.
Hours after the arrests, Japanese media said the offices of another lawmaker from Mr Abe's Liberal Democratic Party, Takaki Shirasuka, had been searched as part of the same investigation.
The government regards the opening of casinos as a way to boost Japan's flagging economy.
In 2016, after years of fractious debate, parliament voted to allow casinos to operate within hotel and conference facilities. However, no casino licences have so far been issued.
Gambling has a seedy image in Japan, and opinion polls have suggested that most people there remain opposed to the opening of casinos.