An electric linear motion actuator comprising: an electric motor having a rotor shaft; a reduction gear mechanism configured to reduce the speed of, and output, rotation of the rotor shaft of the electric motor; a slide member capable of axially moving along a center axis of an output gear in the reduction gear mechanism; and a rotation-linear motion converting mechanism (10, 24) configured to convert a rotary motion of the output gear to a linear motion and transmit the linear motion to the slide member; and a locking mechanism capable of locking and unlocking the rotation of the rotor shaft of the electric motor; wherein the locking mechanism comprises: a plurality of locking portions provided in a side surface of one of a plurality of gears constituting the reduction gear mechanism so as to be arranged in a circumferential direction of the one of the plurality of gears; a locking pin configured to be moved forward and backward with respect to the locking portions, and brought into engagement with any one of the locking portions to lock the one of the gears when the locking pin is moved forward; and a linear solenoid (47, 49) configured to move the locking pin forward and backward; characterized in that a plunger of the linear solenoid and the locking pin are formed as separate parts, and disposed coaxial with each other such that an end surface of the plunger and an end surface of the locking pin axially face each other; the locking pin is made of a non-magnetic material; and a return spring is provided to apply an elastic force to the locking pin, thereby allowing the plunger to move backward along with the locking pin. an electric motor having a rotor shaft; a reduction gear mechanism configured to reduce the speed of, and output, rotation of the rotor shaft of the electric motor; a slide member capable of axially moving along a center axis of an output gear in the reduction gear mechanism; and a rotation-linear motion converting mechanism (10, 24) configured to convert a rotary motion of the output gear to a linear motion and transmit the linear motion to the slide member; and a locking mechanism capable of locking and unlocking the rotation of the rotor shaft of the electric motor; wherein the locking mechanism comprises: a plurality of locking portions provided in a side surface of one of a plurality of gears constituting the reduction gear mechanism so as to be arranged in a circumferential direction of the one of the plurality of gears; a locking pin configured to be moved forward and backward with respect to the locking portions, and brought into engagement with any one of the locking portions to lock the one of the gears when the locking pin is moved forward; and a linear solenoid (47, 49) configured to move the locking pin forward and backward; characterized in that a plunger of the linear solenoid and the locking pin are formed as separate parts, and disposed coaxial with each other such that an end surface of the plunger and an end surface of the locking pin axially face each other; the locking pin is made of a non-magnetic material; and a return spring is provided to apply an elastic force to the locking pin, thereby allowing the plunger to move backward along with the locking pin. a plurality of locking portions provided in a side surface of one of a plurality of gears constituting the reduction gear mechanism so as to be arranged in a circumferential direction of the one of the plurality of gears; a locking pin configured to be moved forward and backward with respect to the locking portions, and brought into engagement with any one of the locking portions to lock the one of the gears when the locking pin is moved forward; and a linear solenoid (47, 49) configured to move the locking pin forward and backward; characterized in that a plunger of the linear solenoid and the locking pin are formed as separate parts, and disposed coaxial with each other such that an end surface of the plunger and an end surface of the locking pin axially face each other; the locking pin is made of a non-magnetic material; and a return spring is provided to apply an elastic force to the locking pin, thereby allowing the plunger to move backward along with the locking pin.