A former top bureaucrat accused of murdering his socially reclusive son last year entered a plea of not guilty on the grounds of self-defense at the first hearing of his appeal trial Tuesday.

During the hearing at the Tokyo High Court, the defense team for Hideaki Kumazawa, a former vice farm minister, stated that "Since his intuition told him he could be killed by the victim, he killed him by reflex."

Kumazawa, 77, was sentenced to six years in prison by the Tokyo District Court in December last year.

Although the lower court ruling said Kumazawa had premeditated the murder of his 44-year-old son Eiichiro, his defense said Tuesday the ruling was based on factual errors.

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"After his eldest son threatened him by saying 'I'll kill you,' (the defendant) committed the crime because he believed he had no choice but to resist by stabbing his son with a knife," a defense lawyer said.

The defense also argued that even if his conduct did not constitute self-defense, Kumazawa is not liable in the case as he believed he was in a crisis.

Failing acquittal, Kumazawa should have only a suspended sentence as his son's violence at home was the cause of the killing, the defense said.

The district court found that Kumazawa stabbed his son in the neck and chest multiple times on June 1 last year in their house in Tokyo, causing his death from massive blood loss.

While the defense had sought a suspended sentence, claiming Kumazawa had long supported his son who had a developmental disorder, the lower court also acknowledged that the former bureaucrat had "the strong intention to kill."

The case has drawn public attention due to the large number of social recluses, known as "hikikomori," with the government putting the figure of such people aged 40 to 64 in Japan at around 613,000 across the country.

Kumazawa joined the farm ministry in 1967 and became vice minister in 2001, before serving as Japan's ambassador to the Czech Republic from 2005 to 2008.