In a child seat comprising a base unit fixed onto a seat of a vehicle by using a seat belt of the vehicle and a seat unit provided with a seat cushion portion and a seat back portion and fixable onto the base unit in a forward attitude with respect to the seat, a waist belt portion of the seat belt can be inserted between the base unit and the seat cushion portion in a lateral direction of the seat, and further, a waist belt guide for inhibiting the movement of the base unit forward of the seat by hooking the waist belt portion is provided in the base unit so as to hold the waist belt portion at a position nearer a fore end of the base unit.