PURPOSE: To make it possible to operate the titled system in a stabilized condition by maintaining the interior of the system at a constant pressure by a method wherein in an arrangement in which a supply water tank is connected to a jet pump in a bypass passage of a circulation pump through a check valve, a flow rate control valve is arranged between the check valve and the tank.

CONSTITUTION: A heat exchanger 25 disposed in a heat accumulating tank 23 and a solar heat collector 24 are connected to each other through a feed pipe 27 provided with the circulation pump 26 and a feedback pipe 28. Further, a bypass passage 31 provided with the jet pump 30 are connected between the discharge side and the sucking side of the circulation pump 26 and the pump 30 is connected with the supply water tank 34 through the check valve 32. In the above system, the flow rate control valve 33 is interposed between the check valve 32 and the supply water tank 34. In operation, when the pressure in a pressure switch 29 reaches a set pressure at the time of circulation of water, the valve 33 is closed through a control box 40 until the pressure in the system drops, to thereby maintain the interior of the system at a constant pressure.