"I picked it up because I thought it was seashell. I then found that it might be a grenade but thought I shouldn't leave it there. I didn't think it might explode," he told reporters.

The student picked up the object, corroded and covered with seaweed, on the shore in Yomitan on Okinawa Island on March 16 and he checked his baggage containing the object on a Japan Transocean Air flight from the Okinawa capital Naha to remote Ishigaki Island on Saturday.

The airline said it will look into the matter and review its safety check system.

He later took the object to Taketomi Island from Ishigaki Island by ship.

The object is about 10 centimeters long and up to 6 cm in diameter, according to the police.

The operator of a Taketomi Island guesthouse, where he was staying, told the police Sunday that the student was carrying the object.

The object was put in a field near the guesthouse and will be collected soon by a bomb-disposal squad of the Ground Self-Defense Force.