To restrain partial abrasion of an outer peripheral side pad in a double-drum type drum brake.

A frictional force generating device 10 is arranged between both cylindrical parts 14 and 16 of a double drum 12 having the outer peripheral side cylindrical part 14 and the inner peripheral side cylindrical part 16. The frictional force generating device 10 has the outer peripheral side pad 52 to an inner peripheral frictional surface 50 of the outer peripheral side cylindrical part 14, an inner peripheral side pad 56 to an outer peripheral frictional surface 54 of the inner peripheral side cylindrical part 16, and a cylinder assembly 24 for driving both these pads. The cylinder assembly 24 has first and second outer peripheral side cylinders 30 and 32 and an inner peripheral side cylinder 34. The first and second outer peripheral side cylinders 30 and 32 are not symmetrically arranged to the center in the peripheral direction of the outer peripheral side pad 52, and are deviatively arranged to the downstream side in the rotational direction of the double drum 12. At the same time or instead of it, a diameter of a piston 42 of the first outer peripheral side cylinder 30 on the downstream side, may be formed larger than that of the second outer peripheral side cylinder 32.