PURPOSE: To keep the internal pressure of an optical connection section by adjusting the cooling liquid pressure to match the preset liquid pressure.

CONSTITUTION: When temperature rises and the volume of a cooling liquid 7 is expanded and the liquid pressure becomes larger than the preset liquid pressure, a pinion 23 is rotated counterclockwise, a rack 22 is moved to the right, a piston 32 is moved to the right, the volume of a pressure adjusting chamber 33 is increased, and the cooling liquid pressure is returned to the preset pressure. When temperature is lowered and the pressure of the cooling liquid 7 becomes smaller than the preset liquid pressure, the pinion 23, rack 22 and piston 32 are all moved in the opposite direction to the above, the volume of the chamber 33 is decreased, and the liquid pressure is returned to the preset liquid pressure. The pressure of the cooling liquid 7 is invariably detected by a pressure sensor 10, feedback control is performed to correctly match the liquid pressure with the preset liquid pressure, thus the internal pressure of an optical connection section can correctly be kept at the preset pressure.