A support mechanism for an opening and closing member slidably moving to open and close an opening formed in a vehicle body, comprising: a support frame adapted to be arranged at the opening and closing member to support the opening and closing member relative to the vehicle body; a guide rail adapted to be arranged at the vehicle body along a side edge of the opening and including first and second guide portions (10d, 10e) that are parallel to each other; and a guided member (8, 51) including a cylindrical portion (33, 54) and a bearing portion (31, 52) continuously formed with the cylindrical portion (33, 54) and coaxially supporting a shaft portion (6, 22) that is arranged at the support frame, the cylindrical portion (33, 54) having an outer peripheral surface (33b, 54b) formed into a tapered shape and contacting the first and second guide portions (10d, 10e) in a biased manner characterized in that the bearing portion is formed into a cylindrical shape and provided in an inner peripheral side of the cylindrical portion, wherein the cylindrical portion includes a first end portion and a second end portion oppositely arranged in an axial direction of the bearing portion and the first end portion having an outer diameter smaller than an outer diameter of the second end portion is continuously formed with the bearing portion, the shaft portion includes a small diameter portion having a predetermined outer diameter and a large diameter portion arranged side by side relative to the small diameter portion in the axial direction and having an outer diameter larger than the predetermined outer diameter of the small diameter portion, the cylindrical portion is configured so that the second end portion having the outer diameter larger than the outer diameter of the first end portion protrudes further along the axial direction than a first end portion (31 c) of the bearing portion,and wherein the small diameter portion of the shaft portion is supported by the bearing portion coaxially in the axial direction, and the second end portion of the cylindrical portion, protruding further than the first end portion of the bearing portion is radially outwardly adjacent to the large diameter portion of the shaft portion. a support frame adapted to be arranged at the opening and closing member to support the opening and closing member relative to the vehicle body; a guide rail adapted to be arranged at the vehicle body along a side edge of the opening and including first and second guide portions (10d, 10e) that are parallel to each other; and a guided member (8, 51) including a cylindrical portion (33, 54) and a bearing portion (31, 52) continuously formed with the cylindrical portion (33, 54) and coaxially supporting a shaft portion (6, 22) that is arranged at the support frame, the cylindrical portion (33, 54) having an outer peripheral surface (33b, 54b) formed into a tapered shape and contacting the first and second guide portions (10d, 10e) in a biased manner characterized in that the bearing portion is formed into a cylindrical shape and provided in an inner peripheral side of the cylindrical portion, wherein the cylindrical portion includes a first end portion and a second end portion oppositely arranged in an axial direction of the bearing portion and the first end portion having an outer diameter smaller than an outer diameter of the second end portion is continuously formed with the bearing portion, the shaft portion includes a small diameter portion having a predetermined outer diameter and a large diameter portion arranged side by side relative to the small diameter portion in the axial direction and having an outer diameter larger than the predetermined outer diameter of the small diameter portion, the cylindrical portion is configured so that the second end portion having the outer diameter larger than the outer diameter of the first end portion protrudes further along the axial direction than a first end portion (31 c) of the bearing portion,and wherein the small diameter portion of the shaft portion is supported by the bearing portion coaxially in the axial direction, and the second end portion of the cylindrical portion, protruding further than the first end portion of the bearing portion is radially outwardly adjacent to the large diameter portion of the shaft portion.