An M-channel (M is an integer of at least two) synchronous rectification type step-down DC/DC converter is provided. A controller in the converter (i) calculates a load current on a basis of currents flowing through M inductors, (ii) dynamically changes the number K of driving phases (K is an integer of up to M) on the basis of the calculated load current, (iii) generates a pulse signal adjusted in duty ratio such that an output voltage of an output line coincides with a predetermined reference voltage, (iv) selects K drivers among M drivers, and distributes the pulse signal with a phase difference of (360/K) degrees to each of the selected K drivers, and (v) monotonically increases an amplitude control signal indicating the amplitude of a gate driving voltage with respect to the calculated load current in a range determined in advance for each number K of driving phases.