PURPOSE: To realize the smooth circulation of heat transfer medium by a structure wherein the cross-sectional area of the heat transfer medium filler port of a liquid reservoir is made smaller than that of the liquid reservoir main body in order to sufficiently perform the syphon function of the liquid reservoir.

CONSTITUTION: The diameter R1 of the filler port 11a is made smaller than the diameter R2 of the liquid reservoir main body 11. Accordingly, the smaller the cross-sectional area or diameter R1 of the filler port 11a is, the greater the rising speed of the liquid level becomes, resulting in easily causing the liquid level to exceed the height of a syphon tube 12 and consequently the syphon tube 12 to sufficiently fulfill the syphon function. When the syphon tube 12 sufficiently performs its function as a syphon, the smooth circulation of the heat transfer medium liquid 10 is realized.