Abstract: A built-in child seat for an automotive vehicle is accommodated within a recess portion of a seat back of a bench type automotive seat. The child seat may be disposed in a retracted or unfolded position and is constructed such that, in an unfolded condition, wing portions are disposed on either side of the child seat cushion for stabilizing an occupant of the seat against laterally applied forces. Further, a T-shape chest support portion is rotatably hinged to a forward portion of the seat cushion to be upwardly oriented for stably supporting a seat occupant's body and legs with simple and convenient structure. Belt adjusting mechanisms are provided for allowing the child seat to be adjusted according to the size of the seat occupant.