PURPOSE: To obtain a filter holding mechanism where the occurrence of distortion caused by the heat of a filter is restrained.

CONSTITUTION: In an illuminator 7, a vitreous filter 2 is held on the front surface of a light source 1 constituted of a lamp 1a and a reflection mirror 1b by a holding part 6 fixed to the aperture part of a frame 3 with a screw 8. The holding part 6 is formed out of light transmissive material such as resin. Then, a recessed part is formed on one side of the four-side state holding part 6 and the end of the filter 2 is inserted in the recessed part. A gap is provided between the end face of the filter 2 and the wall surface part of the recessed part of the holding part 6, so that the filter 2 is fixed to the frame 3 in a state where it has play as a whole. Since the filter 2 is formed out of the light transmissive material, temperature gradient caused by the partial shadow of the filter 2 is prevented from occurring and stress caused by thermal expansion is prevented by the gap in the recessed part of the holding part 6.