PURPOSE: To prevent the inversion of magnetization by projecting light in the one magnetization area of a medium while changing the quantity of light, prior to reproducing information, finding the quantity of light where the ratio of a detection value obtained by photoelectrical conversion to the quantity of light begins to change from a preset value and reproducing the information at the quantity of light smaller than the found one.

CONSTITUTION: A light beam from a semiconductor laser 10 is projected on an optical magnetic disk 19 through a collimate lens 11, a polarizer 12, a beam splitter 13 and an objective lens 14, and a desired reproduction is executed. At that time reflected light from the disk 9 is returned to the splitter 13, reflected by its multilayer, and transmitted to an A/D converter 23 through a photodetecting element 15, a sensor lens 16 and a detector 17. Then a controller 24 controls the output of a laser driver 25. In this constitution, laser light is projected on the disk, prior to reproduction, the projected quantity of light is decided, and a reproduction action is attained through the use of said quantity.