Japan's Prime Minister Kishida Fumio has announced his eldest son, Shotaro, who serves as his secretary, will be replaced to take responsibility for holding a party at the prime minister's official residence last year.

Kishida told reporters on Monday that Kishida Shotaro will resign on June 1. He will be replaced by Yamamoto Takayoshi, who previously held the post until last October.

Shotaro had received a stern warning from his father after facing criticism for holding a party in the prime minister's official residence.

A weekly magazine reported last week that he had held a year-end party with relatives last December. The participants took photos in a room typically used to receive foreign dignitaries. They also held a mock news conference there.

Kishida explained that his son's behavior at the official residence was inappropriate, adding that Shotaro was also a public figure as his secretary.

The prime minister also said he made the announcement after most of the coordination work concerning the Group of Seven summit held earlier this month was complete.

Kishida said he takes the case seriously as he was responsible for appointing his son to the post.

Opposition parties have been demanding Kishida sack his son.

Ohsaka Seiji, the executive deputy president of the main opposition Constitutional Democratic Party, told NHK it was wrong for the prime minister to appoint his son, as he was not qualified for the post in the first place.

Ohsaka said Kishida should have dismissed his son in January, when Shotaro was found to have gone shopping and visited tourist spots using an official vehicle when he accompanied his father's tour of Europe and the US.