A Rankine cycle system has a condenser, a heat pump connected to the condenser, a heat collecting device connected to the heat pump, and an expansion turbine connected to the heat collecting device and the condenser. The heat pump includes an expansion tank or closed vessel, a refrigerant supply conduit connected to the lower part of the expansion tank and to the condenser, and a refrigerant discharge conduit connected to the upper part of the expansion tank. An open/close valve is installed in the refrigerant supply conduit. A pressure regulating valve installed in the refrigerant discharge pipe opens when a pressure reaches a specified value or higher. A temperature regulating device can heat the refrigerant in the expansion tank to produce a refrigerant vapor of saturated temperature or higher, which vapor can be introduced into the heat collecting device.