A side support device in a seat back of an automotive seat, which is mounted at one lateral frame section of a seat back frame within the seat back, is of such a structure that a motor and associated transmissions are disposed adjacently along the inward side of that one lateral frame section of seat back frame, and that a drive mechanism is also disposed along the same one lateral frame section of seat back frame for causing a pair of side support frames to be moved inwardly toward and outwardly away from each other. The drive mechanism includes a lead screw and a nut member in threaded engagement therewith, such that both of them are disposed vertically relative to the seat back frame. Thus, the motor, drive mechanism and related elements are located in the seat back at a point free from contact with the body of a passenger on the seat.