TOKYO (AFP) - Tokyo governor Naoki Inose has admitted that he had received US$500,000 (S$625,700) from a political family at the centre of a dirty electioneering scandal, but denied the money formed a slush fund.

Mr Inose, who is credited for bringing the 2020 Olympics to the capital, told reporters on Friday that the money - which he received before running in last year's gubernatorial election - was a personal "loan" and did not constitute election campaign funds.

Under the Japanese election law, campaign treasurers must report all income, such as donations, related to electioneering.

Mr Inose failed to report the $500,000.

Those who violate the law could face prison terms of up to three years or fines of up to 500,000 yen (S$6,185).

Mr Inose also said at the Friday press conference in Tokyo that he was offered the money by the Tokudas, the family running the powerful medical group Tokushukai.

"I felt it would be rude to refuse when (the money was) offered," he said.

But public broadcaster NHK reported on Saturday that it was Mr Inose himself who reached out to the Tokuda family and asked for 100 million yen before the election.

Mr Takeshi Tokuda, a Lower House member, relayed Mr Inose's request to his father and the founder of the medical group, Mr Torao Tokuda, who then provided 50 million yen, NHK said.

Prosecutors have investigated the Tokushukai group, which runs dozens of major hospitals throughout the nation, over an allegation of illegal electioneering practice, including providing money to campaign workers, at the time the younger Tokuda ran for the lower house.

Mr Inose admitted to receiving the money, which he said he had paid back, after the investigation surfaced in September.

He said he was only able to return the money after the investigation because he had been busy running Tokyo's bid to host the 2020 Olympics, and because he had to attend to his wife, who was hospitalised.