PURPOSE: To upgrade safety by using a heat pipe and providing a leak detector for heat medium at a heat exchanger when liquid is heated with exhaust heat.

CONSTITUTION: The exhaust gas from an engine 1 is discharged outside from a pipe 7 through a primary heat exchager 5. A vaporizing section of heat pipe 14 is inserted into the primary heat exchanger 5, while a condensing section of the heat pipe 14 at the other end is inserted into a secondary heat exchanger 8. As the liquid circulated by a pump 9 flows through the secondary heat exchanger 8, the liquid is heated by exhaust heat and is used for various kinds of heating services in a heater 10. On the other hand, a leak detector is installed in a space between the primary heat exchanger 5 and the second heat exchanger 8. This detector is designed to detect the leakage of heat medium or similar liquid flowing through the secondary heat exchanger 8, thereby preventing accidents by oil leakage.