To solve the problem that a partial discharge occurs during the rotation control of a motor generator.

Operation signals gup, gun, gvp, gvn, gwp, gwn which operate a switching element of an inverter are calculated based on the magnitude relation between command voltages vuc, vvc, vwc and a carrier to a three-phase motor generator. When the command voltage vuc changes from a state larger to a state smaller than the command voltage vvc in a positive region of the command voltages vuc, vvc, a polarity inversion of UV interphase voltage is decided based on a logic inversion of the operation signal gvp on a negative-going edge of the operation signal gup. The surge noise is suppressed by elongating a switching time when the polarity is inverted.