To prevent the whiplash injury of an infant by a child seat attached to a seat part of an automobile seat on which the infant is laid on the back facing the back side of the automobile, and to secure the safety in accordance with the growth of the infant by adjusting the angle of laying the infant and the height of a stool for the infant.

An attaching base 1 comprises a standing part 1A on the front F side of the automobile M and a cross mounting part 1B connected to the standing part 1A sideways and spread on the seat part S. A backrest floor panel part 2 for laying the infant on the back is connected between the standing part 1A and the cross mounting part 1B in the inclined state with the upper part 2a on the front side and the lower part 2b on the back side B. The stool 2 for the infant is disposed orthogonally crossing the backrest surface 2d on the back side B in the backrest floor panel part 2 to be lifted up/down freely. The stool 2 is freely fixed at a required position by the operation of a stool stopping mechanism 4.