A ball seat made of a resin material having a recessed spherical surface and an external peripheral surface, a stud ball having a spherical head portion that engages with the recessed spherical surface in a rotatable manner, and a housing that accommodates the ball seat which is inserted from an opening portion toward a bottom surface, and comprises a ball seat insertion unit where an inner peripheral surface is engaged with an external peripheral surface of the ball seat are provided, wherein an external diameter of the external peripheral surface increases from the bottom surface toward the opening portion, and an internal diameter of the inner peripheral surface increases from the bottom surface toward the opening portion.