To attain the highly accurate detection of the prepit at the time of recording operation.

A sum signal E+F is made out from signals E and F for receiving the return light of the sub-beam which are used in the tracking control of the 3-beam differential push-pull system. This sum signal E+F is inputted to a variable gain circuit 40. The gain of the variable gain circuit 40 is set to the low gain in the mark forming part in accordance with the recording signal and set to the high gain in the blank forming part, then the sum signal E+F is normalized. The normalized sum signal E+F is compared with the appropriate threshold to perform the prepit detection.