PURPOSE: To obtain a compact heat flowmeter by cooling a heat pipe by a natural draft.

CONSTITUTION: When a hot flow strikes a lid 37, electromotive force is generated at a body 40 and a disk 39 forming a thermocouple. This electromotive force is led out through a lead line 43 in a sheath tube 41 and a voltmeter finds heat flux. At the same time, the heat receiving part 34 of the heat pipe 31 near the lid 37 vaporizes the working liquid in the heat pipe 31 to cool the heat pipe and this vaporized working liquid is liquidized by cooling the condensation part 36 of the heat pipe 31 to perform radiation. Consequently, the body 39 at the tip side of the outer cylinder 32 of the heat pipe 31 is cooled.