PURPOSE: To simply adjust the installation position of a detection system by a method wherein a light spot on an optical disk and the state of a track are observer directly.

CONSTITUTION: In an optical pickup optical system 1, a light flux from a laser- diode light source 2 is made a beam of parallel light by using a collimation lens 3, it is reflected by using a beam splitter, and it is reflected by a reflecting mirror 5. The light flux from the source 5 is condensed on a stationary recording face on an optical disk 7 by using an object lens 6, and a light spot and a track guide groove are set to a focused state. A CCD area sensor 17 converts a beam of light flux from a half mirror 15 into a video signal, the image of the light spot by a beam of transmitted light is picked up, and the image of the guide groove is picked up by irradiation with a light source 16 for reflection irradiation use. A monitor 18 displays the light spot on the disk 7 and the track groove. Thereby, the installation position of a detection system or the like can be adjusted simply and easily.