An optical disk apparatus for reproducing or recording and reproducing two or more sorts of optical disks whose recording plane heights are different is arranged to reproduce or record data from or on the optical disk transparently to the user and without the users intricate operations. For this purpose, the optical disk apparatus provides a disk discriminating circuit for discriminating the disks whose recording plane heights are different. The disk discriminating circuit operates to discriminate the disk before reproducing or recording the disk mounted to the optical disk apparatus and switch the optical system and the circuit system to the suitable ones to the discriminated disk and then reproduce or record the data from or on the disk.