To provide a power circuit which is high in the accuracy of detection of the temperature of a heating body to be detected and can be reduced in size.

Two Zener diodes 16 and 17 are connected in series between output terminals 14 and 15. A current detection circuit 18 is connected with the output side of the Zener diodes 16 and 17. The Zener diodes 16 and 17 are disposed so that they are incorporated into a transformer 6. If the transformer 6 is used for a long time and produces heat or if the reverse voltage becomes overvoltage, the emitter-base voltage of a transistor 20 reaches a predetermined value, and the transistor 20 is turned on. Then, a transistor 24 is turned on, and a photocoupler 30 is tuned on as well. As a result, oscillation of a primary-side control circuit 10 ceases, and the power circuit 1 is interrupted.