Eighteen Japanese tourists traveling on a bus were injured Friday together with their driver after the bus went off a highway on Hawaii Island and into a ditch, according to police.

Four of the tourists were seriously injured in the accident off Volcano Highway, including a 76-year-old man and a 34-year-old woman, they said. The man, who suffered head injuries and fell unconscious, was flown to a hospital on Oahu Island along with the woman.

Two others -- a married couple in their 40s from Hokkaido -- were also in serious condition.

The other casualties were treated at Hilo Medical Center in eastern Hawaii. The Japanese Consulate General in Honolulu has yet to identify the tourists.

The bus fell into a 1-meter-deep ditch after trying to avoid a car that had crossed over from the opposite lane. Fire officials said the roads were wet at the time of the accident.

The tour was a one-day trip sponsored by Tokyo-based travel agency JTB Corp. The 19 were going to the popular Hawaii Volcanoes National Park aboard a Jack's Tours bus. The tourists arrived in Hawaii early Friday.