Tokyo - The Japanese high school clerk who sensationally won the Boston Marathon despite having neither a coach nor a sponsor announced on Thursday he was packing in his day job and turning pro.

An overnight media star, Yuki Kawauchi, 31, told reporters waiting for him at Narita Airport: "From April next year, I am thinking about resigning as a civil servant and turning pro. I want to use the (prize) money to do that."

Kawauchi scooped $150 000 when he became the first Japanese man to win the race since 1987, the year he was born.

His victory stunned the marathon community at the prestigious race in Boston, where runners braved the coldest start in 30 years in steady rain and hard wind.

By turning professional, he said he wants to compete with the world's top talents and beat his current personal best time of 2:08.14 - something he has been unable to achieve over the past five years.

School events, like enrolment ceremonies and graduations, have also prevented him from competing in spring-season races, like in Paris and Rotterdam.

"Unless I change my environment, I won't be able to best my personal best. As a Japanese runner, I want to compete with top marathon talents from around the world," he said.

"I saw the Japanese flag flying in the Boston sky. There is nothing like it. I was truly happy. I don't have words to describe how I felt," he said.

The prize money should allow him to focus on training for three to four years, said Kawauchi, whose brother is already a professional long-distance runner.

Marathons are hugely popular in Japan, and Kawauchi is well known for juggling his day job with his running passion.

He has a busy race schedule - he will appear at a half marathon this weekend - because he uses actual races as part of his training.

After briefing journalists at the airport, Kawauchi rushed to his job at Kuki High School in the Saitama region, north of Tokyo.

He pledged to stay on the job for a year to work on various events to celebrate the school's 100th anniversary.