A seat back movement prevention device of a belt-in seat enables a certain prevention of falling down of a seat back at a vehicle collision time of a vehicle mounting a 3-point belt-in seat, the seat back movement prevention device of a belt-in seat is made of one of a belt, a wire, and a chain of which one end side is attached to an upper part of a seat frame disposed in a seat back of a 3-point belt-in seat, and a pretensioner for a seat back being equipped at downward of the seat back, and rewinding one of the belt, the wire, and the chain at an activation time of a pretensioner for a seat belt.