The inner peripheral side radial direction surface of the fixing portion of the dust cover is abutted to the outer peripheral side radial direction surface of the flange portion of the ball seat, and the end portion of the fixing portion is abutted to the side surface of the outer peripheral side of the ball seat. The outer peripheral side radial direction surface of the fixing portion is pressed toward the flange portion by the radial direction surface pressing portion of the die member, and the fixing portion of the dust cover is held by the radial direction surface pressing portion of the die member and the flange portion. The pressing by the radial direction surface pressing portion is appropriately performed to withstand an injection molding pressure of the resin injection.