PURPOSE: To provide an air seal for the central discharge opening of a rotary mill capable of giving uniform pressure over the whole seal part, having a simple constitution and easy in maintenance work.

CONSTITUTION: An annularly worked pipe 15 is fitted to an annular flange 11 placed at the outer periphery of the mill through an annular sliding plate 9 in the vicinity of the central discharge opening of the mill 1 main body. The outer peripheral part of an annular sealant 8a made by laminating a metallic plate 16 and a bimetal 17 is fixed on the side face of a fixing cover 7 covering the periphery of the central discharge opening. The annular sealant 8a is actuated to press the pipe 15 at all times. If the temp. of the contact face of the pipe 15 with the metallic plate 16 is abnormally increased when the mill 1 is rotated, the bimetal 17 is bent in the A direction to generate a space and since the leaked air cools the metallic plate the bimetal is restored to keep sealing. 18 shown in a figure is the supply opening of a lubricant.