A rear wheel steering device of a vehicle, comprising: a housing (1, 1a, 1b) configured to be connected to a suspension mechanism via a first connection member and a second connection member, the suspension mechanism supporting rear wheels (RL, RR) of the vehicle; an electric motor being contained in the housing (1, 1a, 1b); a reduction mechanism being contained in the housing (1, 1a, 1b), the reduction mechanism reducing an output speed of the electric motor; a linear motion mechanism including a nut member and a rod, the nut member being connected to the reduction mechanism and being rotated, the rod being threadedly engaged with the nut member, the rod including a first end portion and a second end portion, the first end portion being connected to the second connection member, the second end portion extending from the nut member and serving as a free end portion; and a nut including a cylindrical portion and a flange portion being formed at a distal end portion of the cylindrical portion, the distal end portion having an enlarged diameter, the nut being threadedly engaged with the free end portion of the rod; characterized in that the linear motion mechanism further comprises an annular member, the annular member being retained at the free end portion of the rod while having clearances (Sa, Sb) between the free end portion of the rod and the annular member in an axial direction and in a radial direction of the rod, respectively, the linear motion mechanism converting a rotational motion of the nut member into a linear motion of the rod, and the rear wheel steering device further comprises a rotation prevention mechanism preventing the annular member from rotating about an axis of the rod relative to the nut. a housing (1, 1a, 1b) configured to be connected to a suspension mechanism via a first connection member and a second connection member, the suspension mechanism supporting rear wheels (RL, RR) of the vehicle; an electric motor being contained in the housing (1, 1a, 1b); a reduction mechanism being contained in the housing (1, 1a, 1b), the reduction mechanism reducing an output speed of the electric motor; a linear motion mechanism including a nut member and a rod, the nut member being connected to the reduction mechanism and being rotated, the rod being threadedly engaged with the nut member, the rod including a first end portion and a second end portion, the first end portion being connected to the second connection member, the second end portion extending from the nut member and serving as a free end portion; and a nut including a cylindrical portion and a flange portion being formed at a distal end portion of the cylindrical portion, the distal end portion having an enlarged diameter, the nut being threadedly engaged with the free end portion of the rod; characterized in that the linear motion mechanism further comprises an annular member, the annular member being retained at the free end portion of the rod while having clearances (Sa, Sb) between the free end portion of the rod and the annular member in an axial direction and in a radial direction of the rod, respectively, the linear motion mechanism converting a rotational motion of the nut member into a linear motion of the rod, and the rear wheel steering device further comprises a rotation prevention mechanism preventing the annular member from rotating about an axis of the rod relative to the nut.