A control method for a polyphase step-up converter that drives step-up circuits, the step-up circuits being connected in parallel between an input terminal and an output terminal from which an output voltage supplied to a load circuit is output. The control method includes, in changing the frequency of each of driving signals from a first frequency to a second frequency while N circuits are being driven, determining whether the second frequency is M times as high as a resonance frequency of a resonance circuit provided between the polyphase step-up converter and the load circuit; and when the second frequency is M times as high as the resonance frequency, setting a phase difference between driving signals to a phase difference determined in accordance with a value of M.