Tetsuya Yamagami, 45, was handed a life sentence on January 21 after he had admitted shooting Japan's longest-serving prime minister with a homemade firearm during an election campaign speech in July 2022. Abe's violent death caused worldwide shock.
Yamagami was overpowered and arrested in front of cameras after the killing.
Yamagami's lawyers had asked the Nara District Court for a fixed-term sentence, citing hardship in his upbringing, according to Japanese media reports.
"We filed a notice of appeal to obtain a chance to correct the lower court ruling following discussions with the defendant," chief defence counsel Masaaki Furukawa said in a statement, according to news agency Kyodo.
According to the reports, the appeal was filed with the Osaka High Court on Wednesday, two weeks after the sentencing.
Yamagami had reportedly told investigators that he acted out of hatred for the controversial Unification Church, which was founded in South Korea in 1954 by staunch anti-communist Sun Myung Moon, and supported in Japan by Abe's grandfather, former prime minister Nobusuke Kishi.
Yamagami's mother had donated large sums of money to the religious organisation, which had left the family in financial ruin.
Abe's assassination brought to light the connections between the Unification Church and members of his ruling Liberal Democratic Party.
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