PURPOSE: To prevent the internal pressure fluctuation of an optically coupling part by throwing an ball, where air is sealed with an elastic film, into the optically coupled part filled with coolant.

CONSTITUTION: A lens integrated projecting tube is equipped with a projecting tube 1 and the first lens and a group of lenses 6, and an optically coupling part for them is composed of a bracket 5 and o rings 4 arranged between this bracket 5 and the projecting tube 1 and the lens 8. And the optically coupling part is filled with coolant 7, and an air ball 2, where air 3 is sealed into spherical shape by an elastic film, is thrown in. According to this constitution, even if the temperature of the coolant 7 rises and the volume of the coolant 7 expands, the air ball 2, where the air is sealed with an elastic film, shrinks by that amount accompanying the pressure rise of the coolant 7. Hereby, the internal pressure of the optically coupling part can be maintained constant.