PURPOSE: To provide the optical block attaching and detaching mechanism equipped with a function which fixes an optical block with a simple constitution and a shutter function which protects an operator from incident light harmful to the human body.

CONSTITUTION: The optical block attaching and detaching mechanism consists of a cassette female dovetail 107, a housing 102 which has an optical block fixed member 108 having a male dovetail depression surface 110, a male screw 111, and a shutter hole 114, and a cassette 123 which fixes the optical block 105 and has a cassette male dovetail 106; and the cassette male dovetail 106 is inserted into the cassette female dovetail 107, the male dovetail depression surface 110 presses the cassette male dovetail 106 by the rotation of the optical block fixed member 108 and places a shutter hole 114 at an opening position, and as the depression by the male dovetail depression surface 110 is removed, the shutter hole 114 is placed at a cutoff position.