the present invention discloses an improved rigid structure of a cushion frame provided with a vehicle seat for a child , wherein the seat for a child is mounted at the rear side of a seat back . further , a seat belt anchor of the seat is firmly fixed to the cushion frame . to a back frame member of a cushion frame to which a seat belt anchor for a child is fixed , a horizontal portion of a reinforcing member having a large section modulus in the longitudinal direction is firmly fixed . further a vertical portion of the reinforcing member having a large section modulus in the vertical direction is firmly fixed to the right and left frame members of the cushion frame so as to reinforce the back frame member and the right and left frame members of the cushion frame .