PURPOSE: To facilitate the supplying and monitoring of heat medium fluid and at the same time eliminate the need for installing an expansion tank on rooftop and consequently simplify the roofing work by a structure wherein a heat medium tank and an expansion tank are made into an integral body.

CONSTITUTION: A piping 4a at the outlet side of a heat collecting pipe is divided into a piping 4b connected to the inlet of a heat exchanger 2 and a piping 4c connected to the bottom of a heat medium tank 7. A four-way valve has connection ports A, B, C and D, which are respectively connected to a piping 4a at the outlet side of the heat exchanger 2, a piping 4e at the inlet side of a circulating pump 3, a piping 4f, which is connected to the bottom of the heat medium tank 7, and a piping 4g, which is open at the upper part of the heat medium tank 7. The heat medium tank 7 acts as both an expansion tank and a heat medium tank.