A lock mechanism is provided in an actuator capable of changing a toe angle of a wheel by rotational drive of a motor and adapted to lock operation of the actuator when the rotational drive of the motor is stopped. The lock mechanism includes a casing secured to a housing of the actuator, an input-side shaft connected to a motor shaft of the motor and rotatably held in the casing, an output-side shaft to which rotational torque is transmitted from the input-side shaft, and an engaging part. The engaging part includes a pair of friction members displaceably provided along a guide groove, a claw part having an acute shape in cross section and a side end part which are provided on the input-side shaft, an abutting part provided on each friction member, and a coil spring for urging each friction member.