The disclosure relates to a semiconductor device and a step-down multi-phase DC/DC converter. A DC/DC converter having higher load response performance and efficiency is provided. The step-down multi-phase DC/DC converter includes multiple output-stage circuits, which generate multiple switch voltages in rectangular waves by means of switching an input voltage, and an output voltage is obtained by means of rectifying and smoothing the multiple switch voltages. An error voltage is generated on the basis of a feedback voltage corresponding to the output voltage and a reference voltage, multiple feedback pulsating voltages variant with the multiple switch voltages are generated on the basis of the feedback voltage, and an on timing sequence including multiple on timings is generated according to the generated voltages. The multiple output circuits are switch-driven sequentially according to the on timing sequence, so as to set phase differences for the switch-driving.