PURPOSE: To arbitrarily use plural kinds of optical disks in different recording density by equipping plural objective lenses which are different in specifications and selectively using an objective lens suitable for a loaded optical disk.

CONSTITUTION: When a disk cartridge 1 is inserted into the device, the presence of a hole 1c is detected by a cartridge hole sensor 7. Then, in the case of standard specification cartridge without the hole 1c, a mirror driving device 6 is driven to form an optical path between a fixed optical system 2 and a polarization prism 35 by reflecting a laser beam with a mirror 5 by using an objective lens 32. On the other hand, when a nonstandard specification cartridge with the hole 1c is inserted, the mirror 5 is turned at 45 degrees and the laser beam is reflected with a mirror 4 by using an objective lens 31 to form an optical path between the optical system 2 and a polarization prism 34. Consequently, either of the standard and nonstandard disk cartridges can be used.