Apparatus and method for exposing an optical disk master having a laser source, a deflector for deflecting a recording laser beam based on the laser beam of the laser source and an objective lens for focusing the recording laser beam on an optical disk master, including a liquid crystal plate provided between the laser source and the deflector, a parallel flat plate provided between the deflector and the objective lens, a photodetector that detects reflected light from the parallel flat plate, and an adjusting device for adjusting a voltage applied to the liquid crystal plate based on information on wavefront fluctuation of the recording laser beam detected by the photodetector.