PURPOSE: To reduce the stress applied on a window plate by making a frame part excellent in appearance without a sink mark, etc., on its outer surface without requiring a high pressure at the time of injection molding.

CONSTITUTION: The outer margin of a window plate 1 is cast by a synthetic resin made frame part 2, an internal cavity 3 continuing along the frame part 2 is formed in the inside of the frame part 2, and the internal cavity 3 is formed free to ventilate to the outside through a through hole 3a which is smaller than the internal cavity 3.