PURPOSE: To lead discharge gas into a radiator in case of high temperature, bringing its compressor cooling function into full play, and make a refrigerant dischargeable directly to the inside of a case in case of low temperature, by attaching a bypass valve, consisting of a platelike form memory alloy, to part of a discharge silencer.

CONSTITUTION: A discharge silencer 32 and a vent hole 30a to be interconnected to an internal space of a case 1 are installed in a platelike discharge valve cover 30, and a bypass valve 31 formed with a plate material of a reversible form memory alloy in a form capable of closing the hole 30a from one side is engaged there by a bolt 34. At normal temperature or less, as shown in a full line, the bypass valve 31 is of a convex form to the discharge valve cover 30 so that high temperature and high pressure refrigerant gas 20b out of the vent hole 30a is directly discharged to the inside of the case 1, keeping off its overcooling, thus any drop in cooling capacity is preventable from occurring. On the other hand, in time of high temperature, the bypass valve 31 comes to a concave form, closing the vent hole 30a, so that the high temperature and high pressure refrigerant gas 20 is led into a radiator 21 by way of a connecting pipe 18, bringing its compressor cooling function into full play.