An object is to miniaturize booster coils used in a vehicle-mounted booster converter. In the design method for a vehicle-mounted multi-phase converter including multiple booster coils and a switching circuit for generating an induced electromotive force at each booster coil by switching of current flowing to each booster coil for applying an output voltage, based on an input voltage and the induced electromotive force generated at each booster coil, to a vehicle drive circuit, a coupling factor indicating the extent by which the induced electromotive force in one of multiple booster coils contributes to the voltage between terminals of another booster coil is determined on the basis of a relationship between the coupling factor and current ripple component of each booster coil.