PURPOSE: To stabilize a exposing laser beam by comparing a detective level from a detection element with an external signal and controlling light quantity so as to be constant in the case of exposing so as to form a mandering groove on a resist plate.

CONSTITUTION: The laser beam outgoing from an Ar (+) laser 1 passes through a light quantity modulation element 2, a pulse modulation element 3 and a deflection element 4 for imparting a deflection angle to the laser beam for exposing the mandering groove. Then, the beam diameter is expanded by a beam expander 5 expanding the laser beam to full effective diameter of an objective lens 6, and is converged on the resist plate 7 rotating and moving horizontally by the objective lens 6, and is exposed. As a means stabilizing the exposing laser beam, the laser beam after passing through the deflection element 4 is sampled by a beam splitter, etc., and a sample beam is picked up, and the light quantity of the sampled beam is detected by the detection element 9. By the detected level, the laser quantity is controlled by the light quantity modulation element 2.