PURPOSE: To suppress the increase in temperature of enclosed hydraulic fluid and to expand the extent of applications, by arranging a heat insulating material only at the outer part of the heat receiving part at the tip of a heat pipe, which is provided with a sheath pipe wherein a lead wire is inserted and a lid for tip part, which constitutes a thermocouple.

CONSTITUTION: A heat pipe is divided into a evaporating part 12, wherein internal hydraulic fluid 11 is heated to become vapor 16, a heat insulating part 13 wherein the fluid flows, and a condensing part 14. A body and disk 2 of a lid 1 comprise different metals and form a thermocouple. When a heat flow shown by arrows hits the lid 1, the electromotive force generated in the disk 2 and the body 4 is taken out by a lead wire 5 in a sheath pipe 8. The hydraulic fluid 11 is gasified and cooled in the evaporating part 12 in the vicinity of the lid 1. Heat is made to radiate from a fin 9, and, as a result, the body 4 is cooled. When the heat input from the disk 2 is large, the heat removal from the body 4 becomes insufficient. Therefore a heat insulating material 18 is arranged only at the outer part of the body 4 and the heat input to the heat pipe is restricted. Thus the normal operation can be obtained.