PURPOSE: To surely prevent water or the like from entering the inside of a brake drum.

CONSTITUTION: First and second tubular flanges 16, 17 of large and small diameter are formed on an end part 8 of an opening of a brake drum 7, an approximated conical tubular member 20 is provided by fixing a small diameter side 20b to a back plate 9, and inserting a large diameter side 20a in an annular groove 18, an approximately ring-shaped plate member 24 is provided on the large diameter side 20a of the tubular body 20 so as to cross the annular groove 18, and a hole 25 to communicate each face side of the plate member 24 is formed in the lower side of the plate member 24. Because the clearance 26 on the second flange side becomes small, water W or the like is difficult to enter the brake drum 7 even when water W or the like passes through the clearance 22 on the first flange side, and the prevention of water W or the like from entering can be improved. Even when water W or the like passes through the clearance 26 on the second flange side, water W or the like moves downward to reach the tubular member 20,and further flows to the large diameter side 20a and is discharged from a hole 25, and the dischargeability becomes excellent.