With the use of a voltage across a secondary winding of a transformer, a DC output voltage and a conduction time width of a current in the DC reactor on the secondary side circuit in an immediately preceding period, the turning-on time width and the turning-off time width of a synchronous rectification MOSFET in the secondary side circuit of the transformer are obtained by calculations, without receiving any signals from a primary side circuit, to thereby carry out control of a synchronous rectification circuit in a forward DC-DC converter with the time in which a current flows in a diode reduced to the minimum.