A resin housing 7 is injection-molded by using a forming mold 11. Moreover, a concave shaped adhesive reservoir 10 is formed by a sink mark of the resin during injection molding on its inner circumferential surface 7e simultaneously with the injection molding of the housing 7. Accordingly, the adhesive can be prevented from overflowing and the adverse influence caused by the overflowing of the excessive adhesive can be avoided. A decrease in the molding precision of the housing can be also prevented and cost reduction by dispensing with a processing step can be enabled.