A reverse current preventing circuit of an N channel type switching MOS transistor connected between a voltage input terminal and an output terminal to control a conduction state between the voltage input terminal and the output terminal, the circuit comprises: a first MOS transistor connected between a substrate of the switching MOS transistor and a ground point; and a second MOS transistor connected between the substrate of the switching MOS transistor and a point having a piece of predetermined constant potential higher than that of the ground point, wherein the piece of predetermined constant potential higher than that of the ground point is applied to the substrate of the switching MOS transistor while the switching MOS transistor is made to be in its on-state, and ground potential is applied to the substrate of the switching MOS transistor while the switching MOS transistor is made to be in its off-state.