A vehicle speed V is read (step S1), V is compared with a first threshold Vth1 (step S2), and, when V is not less than Vth1, a warning torque is set smaller with increase in V (step S3). When V is less than Vth1, V is further compared with a second threshold Vth2 (step S4), and when V is less than Vth2, the warning torque is set approximately in proportion to the vehicle speed (step S5); when V is not less than Vth2 and is less than Vth1, the warning torque is set at a predetermined constant value (step S6).