The control apparatus for controlling a voltage transforming apparatus having a transformer, power switching elements disposed in a primary side, and synchronous-rectifying switching elements disposed in a secondary side includes a judging circuit making a judgment as to whether or not an output current of the voltage transforming apparatus is smaller than a specified current on the basis of a primary-side current of the transformer and an inhibition circuit inhibiting the synchronous-rectifying switching elements from performing their synchronous-rectifying control operation when the judging circuit judges that the output current is smaller than the specified current. The judging circuit makes the judgment on the basis of the primary-side current flowing through the primary coil of the transformer immediately before the power switching elements are turned off.