PURPOSE: To output an optical beam energy giving the maximum S/N at all times, by controlling the energy of beam irradiated to a recording medium in response to a detecting signal obtained from the detection of the rotating speed of the recording medium.

CONSTITUTION: An output pulse 11 and a pulse 13 from a local oscillator 12 are compared at a phase comparator 14, inputted to a motor driver 16 through a low pass filter 15 to control the rotation of a motor 2 in response to a difference between a prescribed rotating speed and a motor rotating speed. An output pulse 11 of a tachogenerator 10 is inputted to an F-V converting circuit 11 at the same time. An output 18 of the F-V converter 17 is a voltage proportional to the rotating speed of the motor and the recording medium 1, the output 18 is inputted to a driver circuit 19 of a semiconductor laser 5 to control the energy of optical beam irradiated from the semiconductor laser 5 to a value being less than the critical energy Ecr, and to the energy as close as the Ecr.