To provide an inverse connection preventing circuit of a power source wherein it is not necessary to increase the size, the cost is not increased and efficiency is not decreased when consumption current in a load is increased.

This protecting circuit consists of an input terminal 1 to be connected with the positive pole of an external power source, an input terminal 2 to be connected with the negative pole of the power source, load terminals 3, 4 with which a load is connected, a relay 5 having a contact 5a which is normally opened and an exciting coil 5b, and a control circuit 6 for making a current flow in the coil 5b. One out of a part between the input terminal 1 and the load terminal 3 and a part between the input terminal 2 and the load terminal 4 is connected via the contact 5, and the other is connected directly. Only when the positive pole is connected with the input terminal 1 and the load is connected with the input terminal 2, a switch transistor 6a of the control circuit 6 turns on and makes a current flow in the exiting coil 5b, thereby closing the contact 5a.