PURPOSE: To prevent the fact that heat of an illuminating lamp is transmitted to an objective lens, by constituting so that a mounting part of the dark visual field illuminating lamp and a mounting part of the objective lens can be moved independently to each other in the axial direction against the lenz barrel itself.

CONSTITUTION: A lamp mounting cylinder 7 is fitted into the lower part of a lenz barrel 6 so as to be slidable in the upper and lower directions, so that it can be fixed at a desired position with a fixed screw 8. To the lamp mounting cylinder 7, dark visual field illuminating lamps 2 opposed to each other in the diagonal lower direction are fixed, these lamps 2 are connected to an illuminating power source 3, and a lens mounting cylinder 9 is fitted into the upper part of the tube body 6 so as to be slidable in the upper and lower directions and is fixed with a fixed screw 10. Also, to the lenz barrel 6, a cavity part 13 having inlet and outlet ports 11, 12 of cooling air is formed, and moreover, on the outside circumferential surface, a lot of heat disspating fins 14 are formed in one body, which constitutes a radiating means 15.