To provide a step-up/down DC-DC converter which controls the output voltage to be reliably held at a fixed level for an input voltage fluctuation.

The step-up/down DC-DC converter 1 includes a step-down unit 2 having a step-down switch SW1 and delivering an output voltage lower than the input voltage, a step-up unit 3 having a step-up switch SW2 and delivering an output voltage higher than the input voltage, a step-up/down mode switching unit 30 which performs switching between a step-down mode for delivering an output voltage lower than the input voltage and a step-up mode for delivering an output voltage higher than the input voltage, and a control unit 4 which performs on/off control of the step-down switch SW1 by keeping the step-up switch SW2 off during the step-down mode, and performs on/off control of the step-up switch SW2 by keeping the step-down switch SW1 on during the step-up mode. The control unit 4 presets the maximum value of on period when the step-down switch SW1 is switched during the step-down mode, and presets the minimum value of on period when the step-up switch SW2 is switched during the step-up mode.