A vehicle rear wheel steering apparatus comprising: a housing (10, 10x) configured to be supported by the rear wheels via a first connection member and a second connection member; an electric motor that is supported by the housing; a speed reduction mechanism that is connected to an output shaft of the electric motor, and reduces the speed of an output of the electric motor; and a linear motion mechanism that includes a nut member which is connected to the speed reduction mechanism, and a rod which is screwed to the nut member and is connected to at least one of the first connection member and the second connection member, and that converts rotational motion of the nut member into linear motion of the rod, wherein the speed reduction mechanism includes a planetary gear mechanism and characterized by further including: a drive pulley that is connected to the output shaft of the electric motor; a driven pulley which is connected to the nut member, and is supported by the housing such that the driven pulley can be rotated around an axis of the rod; a belt that is provided in a tensed state between the drive pulley and the driven pulley; a sun gear that is rotated integrally with the driven pulley; a ring gear that is non-rotatably supported inside of the housing; a planetary gear that meshes with the ring gear and the sun gear; a columnar pin that rotatably supports the planetary gear and is fixed to the nut member; and a partition wall portion that separates the driven pulley from the planetary gear, and prevents the planetary gear from being axially moved a predetermined distance or longer, and wherein the speed of an output of the electric motor is reduced and the reduced output is transmitted to the linear motion mechanism via the drive pulley, the belt, the driven pulley, and the planetary gear mechanism. a housing (10, 10x) configured to be supported by the rear wheels via a first connection member and a second connection member; an electric motor that is supported by the housing; a speed reduction mechanism that is connected to an output shaft of the electric motor, and reduces the speed of an output of the electric motor; and a linear motion mechanism that includes a nut member which is connected to the speed reduction mechanism, and a rod which is screwed to the nut member and is connected to at least one of the first connection member and the second connection member, and that converts rotational motion of the nut member into linear motion of the rod, wherein the speed reduction mechanism includes a planetary gear mechanism and characterized by further including: a drive pulley that is connected to the output shaft of the electric motor; a driven pulley which is connected to the nut member, and is supported by the housing such that the driven pulley can be rotated around an axis of the rod; a belt that is provided in a tensed state between the drive pulley and the driven pulley; a sun gear that is rotated integrally with the driven pulley; a ring gear that is non-rotatably supported inside of the housing; a planetary gear that meshes with the ring gear and the sun gear; a columnar pin that rotatably supports the planetary gear and is fixed to the nut member; and a partition wall portion that separates the driven pulley from the planetary gear, and prevents the planetary gear from being axially moved a predetermined distance or longer, and wherein the speed of an output of the electric motor is reduced and the reduced output is transmitted to the linear motion mechanism via the drive pulley, the belt, the driven pulley, and the planetary gear mechanism.