A wiper motor has an output shaft that outputs a to-and-fro rotation to swing a wiper pivotally. A transmission mechanism transmits the rotational force from the motor body. A clutch mechanism lets the transmission mechanism run idle not to transmit the rotational force from the transmission mechanism to the output shaft when the output shaft is subjected to a torque larger than an engagement releasing torque. The engagement releasing torque is specified to a value larger than a maximum wiping torque to which the torque applied by the wiper onto the output shaft is estimated to reach during a normal wiping operation of the wiper.