Ichiro Ozawa has denied intentional wrongdoing in the affair
Prosecutors in Japan have questioned one of its most powerful politicians over an alleged funding scandal.
Ichiro Ozawa is considered a kingmaker in the ruling Democratic Party and is known in Japan as the "Shadow Shogun".
Police are investigating whether Mr Ozawa took bribes from construction firms to invest in property.
He submitted himself voluntarily to questioning and later issued a statement insisting he had not received any illegal funding.
Earlier this month three of Mr Ozawa's current or former aides were arrested on suspicion of improperly reporting donations.
But in his statement, Mr Ozawa said: "I have never received illegal money and I am sure neither have my office staffers."
Mr Ozawa, 67, attend the questioning voluntarily at a Tokyo hotel.
He had not been obliged to do so but had come under mounting political pressure.
At issue is the purchase in 2004 of a plot of land in Tokyo for around $4m by Mr Ozawa's political fundraising organisation.
Prosecutors have been investigating the source of the money.
Reports in Japan say they suspect construction firms seeking public contracts may have been involved.
Mr Hatoyama has been standing by Mr Ozawa, who has huge influence in the party.
The BBC's Roland Buerk in Tokyo says Mr Ozawa is a backroom bruiser credited with masterminding the Democratic Party's victory in elections to the lower house of parliament in August. The win ended half a century of conservative dominance.
But our correspondent says the whiff of scandal is undermining government support ahead of elections to the less powerful upper house in July.
The Democratic Party has been dogged by corruption allegations since coming to power.
In December Mr Hatoyama apologised after two former aides were charged with misreporting millions of dollars of donations.