PURPOSE: To make it hard for a rubber plug to be slipped off during the periods it is used, and mak it easy to draw out it during inspection by providing a pin to be inserted into the rubber plug so that it can penetrate through the inside of it, and forming the rubber plug so that the outside diameter of an inner side collar part of the rubber plug becomes smaller in state where the pin is not inserted.

CONSTITUTION: The rubber plug removed when the state of a brake shoe inside a brake drum is inspected, and after inspection it is fitted-in in order to prevent muddy water and dust from entering. In this case, the rubber plug 10 is formed of a cylindrical part 11 externally abutting on the inside periphery of an inspection hole 2, an outer side collar part 12 abutting on the outer side wall of the inspection hole 2 in continuous with the cylindrical part 11, and an inner side collar part 13 abutting on the inner side wall of the inspection hole 2 in continuous with the cylindrical part 11. Further, a metal pin 20 is provided so that it can penetrate through the inside of the rubber plug 10 along the center axis of the cylindrical part 11. The rubber plug 10 is formed so that the outer diameter of the inner side collar part 13 becomes smaller in a state where the metal pin 20 is not inserted. Thus, removal and fitting of the rubber plug 10 for blocking up the inspection hole 2 can be performed easily.