PURPOSE: To obtain a good observing field by arranging an observing device so as to be directly opposed to an objective lens in the image formation side space of the objective lens in an exposure optical system.

CONSTITUTION: The observing device 5 for monitoring the irradiation state of an exposure laser beam emitted from an exposure optical system 2 is arranged on the part directly under the system 2. A lens 6 fixed on the input part of the device 5 is arranged so as to be directly opposed to an exposing objective lens 3 in the image formation side space to be the part directly under the lens 3 in the system 2 and the optical axis of the lens 3 in the system 2 is allowed to coincide with that of the lens 6. Consequently, the true exposed laser beam on the image formation side of the lens 3 can be effectively observed and the position, shape, size, etc., of the exposed laser beam can be accurately adjusted by observing the laser beam in a condition equal to the exposure state of a practical glass master disk.