An apparatus for mounting a child safety seat and a seat for a vehicle, which are capable of easing a restriction on a position of the child safety seat on the vehicle seat due to a limited width of the vehicle, so as to enhance the degree of freedom of such a position. At the same time, the apparatus and seat allow mounting of the child safety seat on the vehicle seat in a precise and reliable manner in such a way that the child safety seat can endure an impact load in a longitudinal direction of the vehicle or a lateral direction of the vehicle.