PURPOSE: To furnish a single-diode current resonance type DC/DC converter which is so constructed that a resonance current having a high peak value does not flow directly to a DC input power source and which is excellent in a circuit efficiency.

CONSTITUTION: In a single-diode current resonance type DC/DC converter, a three-terminal zero current switching resonance type circuit 3 is provided in a primary-side circuit. In the resonance type circuit 3, a semiconductor switch element 7 and a recovery diode 8 connected in parallel to each other and an inductance 13 for resonance having a reverse connection type center tap construction are connected in series between a DC input power source 1 and a primary- side inductance 4 of a high-frequency transformer, and a capacitor 6 for resonance is connected in parallel between the DC input power source 1 and a center tap of the inductance 13 for resonance.