To provide a DC-DC converter capable of reducing a power loss in secondary side elements without accompanying degradation in utilization factor of voltage in a transformer.

A detecting circuit 12 detects a current in the primary winding 4a of a transformer 4. A variable delay circuit 13 controls delay time in the gate signals S1, S2 of switching elements 5a, 5b, in accordance with increase and decrease of a current in the primary winding 4a of the transformer 4 detected by the detecting circuit 12. Consequently, an off-timing can be increased and decreased in the switching elements 5a, 5b.