Japanese teenage golf star Ryo Ishikawa apologised at the airport shortly before leaving for the US Open after he was found to have driven without a valid licence. Japanese media reported on Wednesday that Ishikawa acquired both a US and international driver\'s licence while staying in the United States to play in PGA events between February and April. Ishikawa needed to be abroad for at least three full months to make his international licence valid at home but he was apparently unaware of the rule until late May when he was seen driving a silver Audi to a domestic tournament. \"It\'s my own carelessness. I\'m very sorry for causing a trouble to my sponsors and golf fans with my fault. I\'m going to work hard as a professional golfer by raising consciousness,\" said Ishikawa, 19. \"I will try to finish high at the US Open, but I tell you the truth, I\'m not calm mentally. But it\'s part of my life. I\'m going to concentrate on playing golf,\" he added. According to his spokesman, Ishikawa was questioned by police at home on Thursday and explained how he got the international driver\'s licence in the United States. His father, Katsumi, has told local media: \"I thought I confirmed that the licence would be effective in Japan when we were in the United States, but it was not enough. It\'s good to find it before he causes an accident.\" Ishikawa has been a hot marketing property since May 2007 when he became the the youngest men\'s winner on a major tour by lifting the domestic Munsingwear Open KSB Cup aged 15 years and eight months. He also became the top money earner on the Japan Tour in 2009 and stunned the world in May last year by shooting a record 12-under-par 58 to win a domestic event. With contracts with several big companies including Toyota and All Nippon Airways, his sponsorship deals are estimated at nearly 10 million dollars.