PURPOSE: To cancel the generation of an unnecessary picture caused by reflected light on a boundary face between optical coupling(OC) liquid and air by putting-in a plastic pellet, for which specific gravity is smaller than that of the OC liquid, and floating the pellet on the surface of the OC liquid.

CONSTITUTION: To an OC housing 11, a lens element 13 of a projection lens 12 is fitted on left side and a CRT 14 is fitted on right side. A plastic pellet 19, which is colored in black, is floated on the whole surface of OC liquid (ethylene glycol) 30 to be injected into an OC room 18. Thus, the light is regularly reflected on the boundary face between the ethylene glycol 30 and air and the light enters the projection lens 12. Then, the light is not projected on a screen. Thus, the generation of the reflected light, which is the cause to generate the unnecessary picture, is canceled.