PURPOSE: To achieve reduction of demand by increasing switching speed by a method wherein a minimum intensity of laser beam which can be detected with a positional detection means is made not more than a maximum intensity of laser beam which can not be recorded on a recording medium.

CONSTITUTION: When an image signal synchronizes with a standard positional signal, and output of a RS flip-flop 22 is at a higher level than an image signal control circuit, a modulator drive circuit 26 outputs drive voltage of an optical modulator 2 corresponding to a light quantity level which performs recording on a recording medium 8 to the optical modulator 2 according to voltage of a modulating signal input terminal. When the output is at a low level, the modulator drive circuit 26 outputs drive voltage to the optical modulator 2 corresponding to a bias light quantity level incapable of recording on the recording medium 8 to the optical modulator 2 according to voltage of the modulating signal input terminal. Generally the recording medium 8 is not exposed to light when the intensity of laser beam is not more than a certain threshold value. If a detectable minimum intensity of laser beam of a positional sensor 9 is made not more than a threshold value of the recording medium 8, though laser beam at a bias light quantity level not less than the detection minimum light quantity is radiated at the threshold value or under, the recording medium 8 is not exposed to light. However, the position becomes capable of being detected.