Abstract: The die 18 of the self-piercing rivet fastening device is equipped with a center pin 25 for receiving the hollow section of the legs 5 on the self-piercing rivet 1 and a die main body 27 having a cavity 26 for guiding the distortion of the tip of the legs on the self-piercing rivet outward radially on the outer periphery of the center pin 25. The center pin 25 and the die main body 27 are supported so as to move relatively freely in the axial direction of the center pin 25 towards the punch 14. It also has a plate spring 41 allowing the center pin 25 to move relative to the die main body 27 so the center pin 25 comes into contact with the die end surface of the fastened member 3 on the receiving end when the self-piercing rivet 1 under pressure from the punch 14 is driven into the fastened members 2, 3 and the tip of the legs begins to pierce the fastened member 3 on the receiving end. This reduces or eliminates the constraints on the fastened members in the rivet-driving direction.