To reliably fix a little child to a seat by movably installing a locking projection member in one of left/right safety belts respectively installed in the vertical direction in upper/lower and left/right installing parts and a locking recessed member in the other.

In left/right safety belts 10, 11, both ends are installed in safety belt installing parts 12, 13 arranged on the left/right ends of an upper part and a lower part of a rear surface frame 4. The length of these safety belts 10, 11 can be adjusted by plastic adjusting tools 16, 17. A locking projection member 18 is movably installed on the safety belt 10, and a locking recessed member 19 is movably installed on the safety belt 11. In a state of installing the safety belts by placing a little child on a child seat, the safety belts 10, 11 contact with both shoulders and both waists of the little child, so that the locking projection member 18 and the locking recessed member 19 engage in a belly part of the little child. Thus, the safety belts 10, 11 press in the direction of girth of the little child and the rear surface frame 4 up to both chests and both shoulders from the belly part center to fix the little child to the child seat.