Playback durability of a writable optical disk is ensured. A method of controlling a playback condition includes continuously irradiating an optical disk with a laser beam having a power level lower than a mark formation level and detecting a change of a state of a signal caused by a return light from the optical disk, and setting a playback condition for the optical disk according to the change of the state of the signal. The playback durability of the optical disk can be ensured by adaptively controlling the playback condition as stated above.