An optical disk apparatus includes a spindle motor for rotating the optical disk. The optical pickup device records or reproduces information into/from the optical disk, and a guide shaft 6 guides the optical pickup device in a radius direction of the optical disk. The optical disk is transferred by a disk tray 3. The disk tray and the optical pickup device 7 etc. are contained in a casing 1. A heat storage device 10, a phase of which is changed in an operating state of the optical disk apparatus, is arranged in a casing.