PURPOSE: To prevent the creep of fusible alloys and to enable a fusible plug to sufficiently exhibit its function by connecting one end side of the communication hole of the main body of a coupling to the high-pressure section of a coolant circuit and filling up one end side of the communicative hole with a fusible alloy having a high melting point and the atmosphere side of the whole with another fusible alloy having a low melting point with a hollow layer section in between, and then, connecting the hollow layer section to the low-pressure section of the refrigerant circuit.

CONSTITUTION: A fusible plug 9 having a hollow layer section 84 is provided with a communication hole 81a containing fusible alloys 82 and 83. One end (A) side of the hole 81a is connected to the high-pressure section of a coolant circuit and the other end (B) side is positioned in the atmosphere. The alloy 82 having a relatively high melting point fills up the end (A) side of the hole 81a and the second alloy 83 having a relatively low melting point fills up the other end (B) side, namely, the atmosphere side of the hole 81a. The hollow layer section 84 is formed between the alloys 82 and 83 filling up the communicative hole 81a of the main body 81 of a coupling. A by-pass pipe 9 communicates an accumulator 7 with the hollow layer section 84. An excessively high rise in the high-pressure section that melts the alloys 82 and 83, releases a refrigerant to the atmosphere and causes the breakage, etc., of the refrigerant circuit is prevented from occurring.