A man was arrested Monday on suspicion of sending a threatening letter with a shotgun shell to rookie lawmaker Taro Yamamoto, who breached Imperial protocol last October by handing a personal letter to Emperor Akihito.

Mikio Mizutani, a 55-year-old former president of a Nagoya metal working company, was arrested by Tokyo police on suspicion of sending the threatening letter from Chiba Prefecture to Tokyo in November, sources close to the investigation said.

The letter, which was intercepted by Tokyo postal workers following a routine X-ray examination of incoming mail, read: "In the coming days, I'll shoot you dead."

Post office officials handed the letter over to the police and it was never delivered to Yamamoto.

Mizutani, who has a gun license for shooting clay pigeons, was arrested in February for allegedly sending a bullet and a threatening note to one of his former clients in Nagoya. Both letters had similar wording and handwriting.

Yamamoto, a Tokyoite who made his Diet debut after winning a seat in the Upper House election last year, raised eyebrows by writing a critical letter about the government's handling of the Fukushima nuclear disaster and handing it to the Emperor at a garden party on Oct. 31. The move drew a rare rebuke from the Diet and angered some nationalists, who saw it as disrespectful.