A synchronous rectification type DC-DC converter equipped with a burst mode prevents backward flows of currents on the secondary side thereof. The DC-DC converter turns off a control signal of a switching element on the primary side when the load thereof becomes light, and includes a voltage converting transformer, a first switching element connected to the primary side coil of the transformer, a primary side control circuit performing on-off control of the first switching element, a synchronously rectifying second switching element connected to the secondary side coil, and a secondary side control circuit performing on-off control of the second switching element. A pulse width ensuring circuit ensures that the pulse width of the control signal performing on-off control of the first switching element does not become equal to or less than the original width of a PWM pulse when entering the burst mode and when exiting the burst mode.