A combined heat and power system according the present disclosure includes a Rankine cycle apparatus including an evaporator that heats a working fluid by heat exchange between the working fluid and a heat source medium, an expansion machine that converts expansion power of the working fluid into rotational power, and a condenser that cools the working fluid by heat exchange between the working fluid and a heat medium, and a thermal circuit for using the heat medium heated by the condenser. An expansion volume ratio of the expansion machine is equal to or less than an expansion ratio in a theoretical Rankine cycle determined based on a state of a temperature and a pressure of the working fluid at a discharge port of the expansion machine and a state of a temperature and a pressure of the working fluid at a suction port of the expansion machine.