Optical-beam 2 emitted from semiconductor laser device 1 is transformed into parallel-light 4 by means of collimator lens 3, and a part of parallel-light 4 passed through parallel glass-plate 7 of which the front-surface is mirror finished and the rear-surface is frost finished is inputted into detector 5 monitoring the light-quantity. The optical-beam reflected from parallel glass-plate 7 is focused on information medium 9 after it passed through a collimator-lens 6. The optical-beam reflected from information medium 9 is then introduced on a detector disposed on the semiconductor laser device taking an reversed course. Since no interference of light transmitted through detector 5 is generated in this optical head, a higher linearity of the output of detector 5 can be obtained so that the better stabilized output control of semiconductor laser device 1 by means of laser control circuit 11 can be obtained.