To conveniently and safely prevent adhesion of a cooking object such as fish.

A second gridiron part 13 is preheated with the cooking object F such as fish mounted on a first gridiron part 12 in an initial ignition period for grill burners 9 and 10. After a while after the ignition, the gridiron part 13 is completely preheated, and lifted upward by a thermally-actuated spring 14, so that the gridiron 11 holding the cooking object F is changed over from the gridiron part 12 to the gridiron part 13. At that time, since the temperature of the gridiron part 12 is kept lower than the protein coagulation temperature of the fish, the cooking object F can be prevented from sticking to the gridiron part 12, and since the gridiron 13 is sufficiently preheated, the cooking object F can also be prevented from sticking to the gridiron 13. Thereby, the shape of the cooking object F is never deformed.