<p>An operating method for an on-vehicle electric motor (7) driving an auxiliary oil pump (8) for supplying the hydraulic pressure of ATF to a transmission through a rotating shaft (9) pivotally supported on ball bearings (10) and (11) only when the idling of an engine is stopped. When an automobile is running and the auxiliary oil pump (8) are not in use, the electric motor (7) is rotated at a very low speed or intermittently rotated. By this, the electric motor can be drivingly rotated slightly in the traveling of the automobile to prevent dents from occurring in the ball bearings. This operating method can be applied not only to the operating method for the electric motor driving the auxiliary oil pump but also to an operating method for an on-vehicle electric motor driving driven devices such as a wiper and a power window through a rotating shaft pivotally supported on rolling bearings.</p>
