In a reverse conducting semiconductor device, which forms a composition circuit, a positive voltage that is higher than a positive voltage of a collector electrode may be applied to an emitter electrode. In this case, in a region of the reverse conducting semiconductor device in which a return diode is formed, a body contact region functions as an anode, a drift contact region functions as a cathode, and current flows from the anode to the cathode. When a voltage having a lower electric potential than the collector electrode is applied to the trench gate electrode at that time, p-type carriers are generated within the cathode and a quantity of carriers increases within the return diode. As a result, a forward voltage drop of the return diode lowers, and constant loss of electric power can be reduced. Electric power loss can be reduced in a power supply device that uses such a composition circuit in which a switching element and the return diode are connected in reverse parallel.