A step-up/step-down DC-DC converter for reducing loss caused by activation and inactivation of transistors. An error amplifier included in a control circuit generates an error signal in accordance with voltage difference between an output voltage and a reference voltage. A PWM comparator compares a triangular wave signal and the error signal to generate a control pulse signal having a pulse width that is in accordance with the voltage difference between the output voltage and the reference voltage. A pulse detector monitors the control pulse signal and generates a mode switch signal for switching the operation mode of the DC-DC converter in accordance with a monitoring state of the control pulse signal. A switch provides the PWM comparator with the triangular wave signal or an offset signal, which is generated by adding an offset voltage to the triangular wave signal, in response to the mode switch signal.