A signal control device controls the period of a three-phase signal used to control a three-phase high-voltage converter. An arithmetic unit of the signal control device determines a timing for changing the period for each phase so that the period of the signal for a V phase or a W phase is changed at the point when the phase difference between a U phase and the V or W phase reaches a prescribed phase difference after the period of the signal for the U phase has been changed. A control unit performs control such that the signal period for each phase is changed at the timing determined by the arithmetic unit.