Electric current supplied to the primary side winding of a switching transformer is switched to generate ac power respectively in a plurality of secondary side windings of the switching transformer. A switching control signal is optically transmitted depending on the voltage of dc power obtained by rectifying and smoothing the ac power generated in the secondary side windings of the switching transformer and whether or not the output side of an electronic circuit connected to the secondary side windings of the switching transformer is short-circuited. When the short-circuit of the output side of the electronic circuit is detected, a switching operation is stopped in accordance with the transmitted switching control signal.