An operation mechanism of a side support apparatus adapted to be accommodated in a right and left side support portions provided at a seatback and being movable between a support position and an open position, includes a support structure including a pulley and being movable between the open position and the support position, a first fixing portion including a first guide portion and a second guide portion and fixedly provided at a seatback member, and a drive cable being fixedly attached to the first fixing portion at a first end portion of the drive cable and being engaged with the pulley at an intermediate of the drive cable. The pulley is pushed by a tensile force generated at the drive cable by pulling the drive cable so that the support structure is movable relative to the first fixing portion in a direction toward the support position.