PURPOSE: To provide an image of good quality by preventing a contrast from worsening and a ghost or the like from being generated without increasing cost.

CONSTITUTION: A photoabsorber, coloring a transparent material in black color with transparency left, is arranged in an internal surface of a coupler 15 in a condition into contact with cooling liquid. A material of the photoabsorber, for instance, is acryl, acetate, etc., with high transparency in no color, not affected by the cooling liquid, and a fiber of dyeing this material by a black dye is used. In this way, cost reduction can be achieved more than the conventional black anodic oxide coating or the like. Unnecessary light is almost completely absorbed, to prevent reflection.