PURPOSE: To make the inside pressure of an optical conjunction part same as atmospheric pressure by forming a rectangular window in a part of a bracket and insetting a movable part having a fan-like cross section shape in the window of the bracket in the way that the movable part is freely rotatable on the rivet part of the fan-shape as a center.

CONSTITUTION: A rectangular window is formed in a part of a bracket 5 composing an optional conjunction part which is filled with a coolant so as to support a movable part 2 having a fan-like cross section shape in the way that the movable part become freely rotatable on the rivet part 2b of the fan as a center, and the movable part is inset to the rectangular window of the bracket 5. The movable part 2 thus rotates on the rivet part 2b of the fan-shape as a center corresponding to the volume alteration of the coolant 7, so that the volume alteration of the coolant 7 is absorbed. In this way, the inside pressure of the optical conjunction part is made almost equal to atmospheric pressure and manufacturing is simplified.