A DC-DC converter can prevent intermittent failure of a PWM or driving signal (V.sub.M) when a load becomes light and prevent generation of unusual noise from a transformer. The converter includes a transformer (2) having a primary winding (2a) and a secondary winding (2b), a semiconductor switching element (17) connected to the primary winding (2a), and a control unit controlling turning-ON and OFF periods of the switching element (17) so that an output voltage (V.sub.O) matches a reference value (V.sub.REF). By controlling the turning ON and OFF operation of the switching element (17), a DC voltage exactly equal to the reference value (V.sub.REF) can be supplied to a load (5). The control unit includes an output voltage detecting and adjusting circuit (6) that outputs a command value (V.sub.C) adjusted for the output voltage (V.sub.O) and the reference value (V.sub.REF), a triangular wave generating unit (7), and a comparator (15) that compares the command value (V.sub.C) and the triangular wave (V.sub.T) to generate the PWM signal (V.sub.M) for driving the switching element (17) according to the result of the comparison. The triangular wave generating unit (7) varies the inclination of the triangular wave (V.sub.T) in response to increase and decrease of the command value (V.sub.C).