A DC-DC converter of the present invention includes: an activation circuit which activates a control circuit for controlling a turning on and off of a switching element; a second rectifying and smoothing circuit which supplies an output voltage, as a power supply, to the control circuit, the output voltage being obtained by rectifying and smoothing a voltage in a tertiary winding of a transformer; an overcurrent protection circuit which limits an output current to be supplied to a load when overloaded; an overcurrent detection circuit which detects that the load is in an overcurrent state, and which outputs a detection signal during a period when the overcurrent protection circuit is operated; and an activation current switching circuit which switches outputs from the activation circuit.