A cable winding device is provided, which is for a flexible flat cable routed to a sliding seat slidably supported allowing for forward and backward movement in a longitudinal direction of a fixed rail; a first side end portion of the cable being fixed to an end of the fixed rail corresponding to a first rail end portion; the device including a winding housing portion inside which a second side end portion of the cable is fixed; the winding portion being fixed to a movable body supporting the seat and configured to slide in a forward and backward direction relative to the fixed rail; in a case where the winding portion moves in a forward (or backward) movement direction toward (or away from) the first rail end portion, the cable is wound in the winding portion (or is fed out along the fixed rail from the winding portion).