A rear wheel steering apparatus for a vehicle comprising: a housing that is connected via a first connecting member and a second connecting member to a suspension mechanism which supports rear wheels (RL, RR) of a vehicle; an electric motor that is accommodated in the housing; a planetary gear mechanism that has a sun gear which is connected to an output shaft of the electric motor, a ring gear which is non-rotatably supported inside the housing, a planetary gear which meshes with the ring gear and the sun gear, and a carrier which is connected to the planetary gear, and reduces the output of the electric motor; and a linear motion mechanism that has a nut member which is connected to the planetary gear mechanism and forms the carrier, and a rod which is screwed to the nut member and is connected to the second connecting member, and converts the rotational motion of the nut member into the linear motion of the rod, wherein the planetary gear mechanism includes a circular columnar pin that rotatably supports the planetary gear and is fixed to the nut member, and a locking member (38, 381, 382, 384) that is joined to a tip end portion of the pin and has at least one contact portion (38e, 381e, 382e, 384e) on a surface thereof facing the planetary gear, and wherein in a state where the planetary gear is installed between the contact portion (38e, 381e, 382e, 384e) of the locking member (38, 381, 382, 384) and the nut member, the planetary gear is supported to be capable of rotating around the pin, wherein the locking member (38, 381, 382, 384) has a concave portion (38a, 381a, 382a, 384a) that supports the tip end portion of the pin, characterized in that a plurality of protruding ridges (38d, 381d, 382d, 384d) are provided on an inner circumferential surface of the concave portion (38a, 381a, 382a, 384a) while being equally spaced along a circumference around the pin, with the protruding ridges (38d, 381d, 382d, 384d) extending in an axial direction of the pin, and protruding in a direction toward the axis of the pin; and wherein the contact portions of the locking member (38, 381, 382, 384) are formed of a plurality of protrusions (38e, 381e, 382e, 384e) which are provided in a protruding manner around the pin while being equally spaced along a circumference around the pin. a housing that is connected via a first connecting member and a second connecting member to a suspension mechanism which supports rear wheels (RL, RR) of a vehicle; an electric motor that is accommodated in the housing; a planetary gear mechanism that has a sun gear which is connected to an output shaft of the electric motor, a ring gear which is non-rotatably supported inside the housing, a planetary gear which meshes with the ring gear and the sun gear, and a carrier which is connected to the planetary gear, and reduces the output of the electric motor; and a linear motion mechanism that has a nut member which is connected to the planetary gear mechanism and forms the carrier, and a rod which is screwed to the nut member and is connected to the second connecting member, and converts the rotational motion of the nut member into the linear motion of the rod, wherein the planetary gear mechanism includes a circular columnar pin that rotatably supports the planetary gear and is fixed to the nut member, and a locking member (38, 381, 382, 384) that is joined to a tip end portion of the pin and has at least one contact portion (38e, 381e, 382e, 384e) on a surface thereof facing the planetary gear, and a circular columnar pin that rotatably supports the planetary gear and is fixed to the nut member, and a locking member (38, 381, 382, 384) that is joined to a tip end portion of the pin and has at least one contact portion (38e, 381e, 382e, 384e) on a surface thereof facing the planetary gear, and wherein in a state where the planetary gear is installed between the contact portion (38e, 381e, 382e, 384e) of the locking member (38, 381, 382, 384) and the nut member, the planetary gear is supported to be capable of rotating around the pin, wherein the locking member (38, 381, 382, 384) has a concave portion (38a, 381a, 382a, 384a) that supports the tip end portion of the pin, characterized in that a plurality of protruding ridges (38d, 381d, 382d, 384d) are provided on an inner circumferential surface of the concave portion (38a, 381a, 382a, 384a) while being equally spaced along a circumference around the pin, with the protruding ridges (38d, 381d, 382d, 384d) extending in an axial direction of the pin, and protruding in a direction toward the axis of the pin; and wherein the contact portions of the locking member (38, 381, 382, 384) are formed of a plurality of protrusions (38e, 381e, 382e, 384e) which are provided in a protruding manner around the pin while being equally spaced along a circumference around the pin.