A control system for preventing a sudden change in the passengers' feeling for the acceleration. After determining whether the assistance using the motor is permitted with respect to each of different control modes, an amount of assistance for each mode is calculated. If the output assistance is permitted in any mode, the maximum amount of assistance is selected, and a previous value of the maximum amount of assistance is compared with the current value of the maximum amount of assistance. A shift value is obtained by subtracting a predetermined assistance value from the previous value when the current value is less than the previous value, and the amount of assistance is set to the shift value until the shift value becomes equal to or less than the current value, so as to gradually shift the amount of assistance from the previous value to the current value.