To provide a power supply which can be protected when the voltage of a DC power supply increases excessively or the DC power supply is connected with reverse polarity and can reduce the cost by decreasing the number of circuit components.

Sources of an MOSFET Q1 for reverse connection protection and an MOSFET Q2 for overvoltage protection are connected in common. The power supply comprises a first switching element Tr1 of pnp transistor having emitter-collector inserted between the gate-source of both MOSFETs Q1 and Q2 and an emiter connected with the positive electrode otf a DC power supply Vs through a resistor R0, a Zener diode ZD2 connected between the base of the first switching element Tr1 and the sources of both MOSFETs Q1 and Q2, and a second switching element Tr2 of npn transistor having collector-emitter inserted between the base of the first switching element Tr1 and the sources of both MOSFETs Q1 and Q2 and being turned on when a voltage detected by a voltage detection circuit 3 exceeds a specified level.