An optical disk apparatus which records/reproduces data on/from a plurality of types of optical disks. The optical disk apparatus has a plurality of laser diodes. Two laser light rays emanating from two laser diodes are coupled together by means of a coupling prism. The thus-coupled laser light is radiated onto an optical disk. A photodetector is disposed in close proximity to the coupling prism. The coupling prism guides, to the photodetector, a laser light component of at least one laser light ray of the two laser light to be coupled together, the component having not been radiated onto the optical disk on an optical path. Laser light which is not radiated onto the optical disk on the optical path includes diffracted light other than a main beam emanating from the laser diode or laser beam which has not been subjected to coupling at the coupling surface of the coupling prism. When laser light for a CD is caused to pass through the coupling surface and laser light for a DVD is reflected by the same, the CD laser light which is reflected from the coupling surface or the DVD laser light which passes through the coupling surface is guided to the photodetector. The power of a main beam of the laser light is controlled in accordance with the power of the laser light detected by the photodetector.