The Nagoya High Court on Monday overturned a lower court acquittal of the nation's youngest mayor, declaring him guilty of taking bribes when he was an assembly member in 2013.

Dismissing the earlier decision, the high court sentenced Minokamo Mayor Hiroto Fujii of Gifu Prefecture to 18 months in prison, suspended for three years, and ordered him to pay a ¥300,000 ($2,700) fine.

The 32-year-old mayor's defense said it intended to appeal immediately. Fujii became the youngest mayor in Japan when he was elected in June 2013.

He was charged with accepting bribes to have a water-cleaning system installed at a school in the city between March and April 2013, when he was a member of the Minokamo Municipal Assembly.

While Fujii consistently maintained his innocence, the lower court questioned the credibility of the bribery confession submitted by Masayoshi Nakabayashi, the president of a water supply installation company.

The high court, presided over by Judge Hiroaki Murayama, said that it found the testimony of Nakabayashi, who was convicted of bribery, to be credible.

Upholding the prosecutors' claim that the mayor accepted the bribes totaling ¥300,000, Murayama said the mayor neglected his public duties.

"Although the amount was not so large, it seemed so thoughtless (for the defendant) to commit such a wrongdoing as a person in public office," the judge said.

Fujii said at a news conference that he was "very surprised" and will not accept the ruling.

Nakabayashi, 46, is serving four years in prison for bribery and other charges.

In the lower court ruling, the Nagoya District Court said in March last year that the Nakabayashi's testimony left doubt because it was "unnatural and wavering," and "lacked a sense of realism at its core."