The disclosed device comprises a duty calculator for calculating a duty command value (Dty), a duty limiter for limiting the duty command value (Dty) to a value according to a limit value (L), a current flow monitor for determining that there is an overcurrent if a current value (Idet) flowing through a winding exceeds a predetermined threshold value (Ithr), and a limit value generator for generating the limit value (L). The limit value generator updates the limit value (L) at predetermined time intervals and for a value corresponding to a difference between the threshold value (Ithr) and the current value (Idet) at a time in order to decrease the current value (Idet) during a period in which the overcurrent is determined.