A support mechanism for an opening and closing member slidably moving to open and close an opening formed in a vehicle body, includes 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 that are parallel to each other, and a guided member including a cylindrical portion and a bearing portion continuously formed with the cylindrical portion and coaxially supporting a shaft portion that is arranged at the support frame, the cylindrical portion having an outer peripheral surface formed into a tapered shape and contacting the first and second guide portions in a biased manner.