1. A control system designed to use a plurality of voltage vectors expressing a switching mode for a switching circuit having a switching element per phase of a multiphase rotary machine to thereby control a difference between a current actually flowing in the multiphase rotary machine and a command current value therefor within an allowable range, the plurality of voltage vectors including a zero vector that allows line-to-line voltages in the multiphase rotary machine to be all zero, and a plurality of non-zero vectors that allow at least one of line-to-line voltage in the multiphase rotary machine to be nonzero, the control system comprising: a drive unit configured to switch only one phase of the switching element for each shift of the switching mode to thereby shift, in accordance with a preset switching pattern, the switching mode from a specified one of the plurality of non-zero vectors to the specified one of the plurality of non-zero vectors via the zero vector.