The present invention provides a bumper including: the bumper back beam made of a composite material, which has an arch-shaped collision surface, one side surface of which is formed so as to extend in a box shape from the collision surface, and a flange surface of which is formed so as to be bent outward from the side surface and made of the fiber-reinforced composite material, wherein the flange surface is fastened to a stay which is cut into a plurality of units based on a fastening point; and said stay, which is fastened to the bumper back beam, wherein a fastening hole corresponding to the fastening point is formed as an elongate hole.