The 19-year-old is a huge celebrity in Japan with numerous multi-million dollar endorsements but was left red faced when he was found to have driven on an invalid international permit.
"I humbly apologise for the embarrassment that my carelessness and clumsiness caused," Ishikawa told a news conference before leaving Japan for next week's US Open.
Ishikawa's remarkable success since becoming the Japan tour's youngest winner as a bashful 15-year-old has already earned him a fortune on and off the course.
His bright smile has made him a favourite among Japanese housewives and he was quick to face the media after his run-in with the law.
"As a professional, I must be more vigilant and devote myself to golf," he said, bowing deeply in a black suit and tie.
"To be honest, my mind is still not at ease at what I did. I will put all my energy into golf."
Ishikawa drove a car in Japan before the international permit he had obtained in the United States had become valid.
Japanese golf officials have criticised Ishikawa and said they would hold a meeting to discuss possible punishments after hearing from the player and his father Katsumi.
The player added: "It's my own carelessness. I'm very sorry for causing trouble to my sponsors and golf fans. I'm going to work hard as a professional golfer by raising consciousness.
"I will try to finish high up at the US Open, but to tell you the truth, I'm not calm mentally right now. From now on I'm going to try and concentrate on playing golf."
His father was quoted in local newspapers as saying: "I thought I had confirmed that the licence would be effective in Japan when we were in the United States, but it was not enough. It's good, though, to find out about our mistake before he caused an accident."
Following the deadly earthquake and tsunami in north-east Japan in March, Ishikawa promised to donate all his earnings for this year to relief efforts in the devastated areas.