PURPOSE: To remove the effect of disturbance light to any wavelength, by setting the wavelength of the filter arranged in front of a photodetector corresponding to the arbitrary wavelength of laser beam.

CONSTITUTION: The primary diffracted beam 17 from a laser beam source passes through a lens 23, a mirror 25, a concave mirror 26, a galvanometer 28 and an objective lens 29 to irradiate an eye 24 to be examined. The reflected beam from the eyeground passes through the objective lens 29, the galvanomirror 28, the concave mirror 26, a lens 30, a variable wavelength filter 31 and a lens 32 to be received by a photodetector 33. The variable wavelength filter 31 consists of a prism 35, a stepping motor 36 for controlling the rotation of the minute angle thereof and a detection slit 37 and the wavelength of the filter is automatically altered with the alteration of the wavelength of laser beam.