To exert cloud preventive function by subjecting a photocatalyst membrane on at least one side surface of a plate glass to light irradiation to activate the photocatalyst membrane without exposing UV ray to a human body.

The photocatalyst membrane 3 is formed on at least one side surface of the plate glass 1, and an exciting light source 5 of the photocatalyst is disposed at a suitable place at an end surface 4 of the plate glass or a peripheral part of the other side surface where the photocatalyst membrane is not formed so that the exciting light does not leak from the place, and/or incidence surface is designed so that the exciting light 2 is made incident from the place to the inside of the plate glass to subject the exciting light to total reflection repeatedly in the plate glass to propagate the exciting light in the plate glass.