PURPOSE: To obtain a protective circuit which functions reliably against layer short circuit or load short circuit by detecting the output voltage itself at the output terminal of secondary winding and utilizing the fact that the voltage difference between the output terminals disappears upon occurrence of layer short between the output terminals.

CONSTITUTION: Resistances of resistors 21, 22 are set such that the emitter voltage and the base voltage of a transistor 20 satisfy the following relation under normal state. (Base voltage > emitter voltage)-(1). In that case, the transistor is reversely biased and subjected to no influence in the operation thereof. Upon layer short between output terminals 4, 5, voltages of rectified outputs 30, 31 are equalized. Consequently, a relation (base voltage < emitter voltage)-(2) is satisfied and the transistor 20 is conducted thus causing function of a protective circuit 54. Upon short circuit of the load connected with the rectified output 30, voltage of the rectified output 30 goes zero and the base and emitter voltages satisfy the relation (2) thus causing function of the protective circuit 54.