PURPOSE: To obtain remarkable heat dissipation effect by a structure wherein a heat pipe and a reciprocating part are assembled together in a heat developing device.

CONSTITUTION: When a spot welder spot-welds a stock to be worked, a large quantity of heat transmits to the tip 8 and the holder 7 of the spot welder. A large quantity of the heat is transmitted to the upper portion of a heat pipe 21 and discharged to the outside air through fins 23 due to the boiling and condensation of liquid 24 in the interior of the heat pipe 21. At this time, because the heat pipe vertically reciprocates integral with the holder 7 of the spot welder, the heat dissipation action of the heat pipe becomes especially effective. Concretely, firstly, the acceleration in the backward motion quickly sends back the liquid droplets 24a adhered onto the inner wall of the heat dissipation section of the heat pipe 21 to the heat reception section and consequently exposes the inner wall surface of the heat dissipation section, resulting in enhancing the heat dissipation effect. Secondly, the reciprocating motion causes air flow in the vicinity of the heat dissipation section or the fins, resulting in increasing the heat dissipation effect of the heat pipe.