PURPOSE: To reduce the cost, to make the performance stable and to facilitate the adjustment by coating the reflection film of split prisms simultaneously.

CONSTITUTION: A split prism 18 having a 1st reflecting face and a 2nd reflecting face forming the image of the reflected beam separately from a recording medium 7 and a photodetector 13 placed along the optical path of both the separated reflecting beams are provided to the device. Then the same reflectance is adopted for the coating of 1st and 2nd face reflecting films of the split prism 18 and coated simultaneously. Thus, the dispersion in the reflected film of the prism 18 caused when the reflected beam from the recording medium 7 onto the detector 13 is reduced optically. Thus, it is made easy to form just focusing of beam onto the recording medium 7.