PURPOSE: To dip a cap in a cleaning bath and to clean the entire cap by covering the surface of a low melting point glass exposed on the outer surface of the cap with a protective film for preventing the component of the glass from being dissolved externally.

CONSTITUTION: A light transmitting glass 2 is integrally adhered with low melting point glass 3 to the cap body 1 of a light transmitting cap 10a. In this case, the periphery of the glass 1 inside the body 1 and the surface 3a of the low melting glass exposed on the periphery of a window 6 of the ceiling surface outside the body 1 are plated by nickel or the like, and the surface 3a exposed on the outer surface of the cap is covered integrally with a plating layer 4a, such as nickel-plated layer of a protective film 4 without gap. When the cap 10a is dipped in a cleaning bath and cleaned, the nickel-plated layer of the layer 4a prevents Pb or the like of the components of the glass 3 from being dissolved in the bath due to the direct contact of pure water or the like of the cleaning bath solution.