A DC-DC converter includes a primary side circuit for allowing a switching device to perform on/off operations to output energy in a primary coil of a transformer to a secondary coil. A secondary side circuit rectifies and smooths a voltage outputted from the secondary coil so as to output a DC-DC voltage. A voltage-detecting circuit rectifies and smooths a voltage outputted from a voltage-detecting coil provided in the transformer so as to output the voltage as a detected voltage corresponding to a voltage to be outputted from the secondary side circuit. A control circuit applies to the switching device a pulse control signal used for controlling on/off operation of the switching device according to the voltage outputted from the voltage-detecting circuit. A primary side current-detecting circuit converts a working current in the primary side circuit, the working current corresponding to a current in the secondary side circuit, for performing detection thereof, and for superposing the thus detected voltage corresponding to the working current with the detected voltage outputted from the voltage-detecting circuit so as to compensate for an amount of variation of an output voltage of the secondary side circuit. The variation is attributed to a variation of the working current in the secondary side circuit. A voltage determined by the superposed voltage is applied to the control circuit.