To reduce the number of components of a protection device arranged between a DC power source and a load circuit for protecting a problem which may be generated at the time of inverse connection.

A protection device 10 is provided with a switch part 20 having an nMOS 22 and whose source terminal S and whose drain terminal D are arranged in a current path between a DC power source 70 and a load circuit 80, a diode 24 arranged so as to be electrically parallel with the nMOS 22, an input terminal 28 to which a control voltage Vc is inputted from the load circuit 80 side, and a resistor 26 whose one end is connected to the input terminal 28 and whose other end is connected to a gate terminal G of the nMOS 22. A stable output DC voltage generated by a series regulator 82 of the load circuit 80 is applied through the input terminal 28 and the resistor 26 to the gate of the nMOS 22 as the control voltage Vc.