PURPOSE: To satisfactorily expose a master disk by independently modulating laser beams for ID and for a groove.

CONSTITUTION: The laser beam emitted from an Ar laser 1 is divided into two by a beam splitter and the like and they are divided into an optical system for ID and that for the groove. Light quantity modulators 2a and 2b changing light quantity, pulse modulators 3a and 3b turning on/off the laser beams and deflection modulators 4a and 4b for vibrating the laser beam and exposing a wobble groove are arranged in the respective optical systems. Light quantity modulators for ID and for the groove are modulated so that they come to possess light quantity required for exposure. The pulse modulator for ID is used for switching a format signal and the groove, and that for the groove is used for switching an ID pit and the groove. Since two deflection modulators are driven by one wobble signal, the laser beams for ID and for the groove can be wobbled in the same phase.