NHK has learned that the man arrested on suspicion of throwing an explosive device at Japanese Prime Minister Kishida Fumio had unsuccessfully sued the government over electoral eligibility.

Police are investigating whether the legal issue is related to the suspect's motive.

The prime minister was about to give a speech in support of a by-election candidate at a fishing port in the western city of Wakayama on Saturday when attempted attack occurred.

Kimura Ryuji, who is 24 years old, was arrested on the spot.

NHK learned the suspect filed a damages suit with the Kobe District Court last year that claimed the government prevented him from vying for an Upper House seat.

Court records show that Kimura maintained that the 30 years or older age qualification, as stipulated for candidacy in the election under the public offices election law, is in violation of the Constitution.

He also claimed that an article of the law requiring an Upper House election candidate to make a 3-million-yen deposit, equivalent to about 22,300 dollars, also violates the Constitution.

He demanded compensation from the government worth 100,000 yen, or more than 700 dollars, for what he claimed was psychological suffering after being ruled ineligible to stand in last July's Upper House election.

The court dismissed the case last November, arguing that the age qualification and the deposit system are reasonable requirements for candidacy. Kimura appealed the ruling last December.

A relevant judgment from the Osaka High Court is expected to be handed down next month.