PURPOSE: To maintain automatically a light quantity of a laser light used for recording, at a prescribed level, by providing a transmittivity control means for changing a transmittivity of a transmittivity changing means basing on a detecting signal from a detecting means.

CONSTITUTION: An output signal from a light quantity detecting sensor 11 is inputted to a detecting circuit 13, amplified and corrected, outputted, to a comparator 14, a signal from the detecting circuit 13 is compared with a reference value set in advance, which is inputted from a terminal C, and a signal for showing its shift quantity is outputted to a controlling circuit 15. The controlling circuit 15 outputs a control signal having a suitable level corresponding to this signal, to a driving means 16, and the driving means 16 operates and a position control of a filter 12 is executed. In this way, it is secured that an incident light quantity to the sensor 11 is at a level of a prescribed range. Accordingly, in a recording device 3, a light quantity is detected by the light quantity detecting sensor 11 provided in a zero order diffraction optical path A, and the light quantity is controlled by controlling a position of an optical filter 12 in accordance with said light quantity and varying an optical transmittivity in the optical paths A and B.