A clutch seal comprising: the clutch seal being an annular seal interposed between a clutch plate in a dry clutch and a piston pressing said clutch plate; a middle ring fixed to said piston between an outer ring and an inner ring which are fixed to a clutch housing; and membrane portions (15, 16) constructed by a rubber-like elastic body respectively between said outer ring and the middle ring and between said inner ring and the middle ring, characterized in that said middle ring is arranged at an intermediate position of a stroke in an initial shape of said membrane portion, the outside membrane portion arranged between said outer ring and the middle ring integrally has an outside root portion, a first fold-back portion protruding to the piston side, a straight portion, a second fold-back portion protruding to the clutch plate side and an inside root portion, between an outside fixed portion fixed to said outer ring and an inside fixed portion fixed to said middle ring, in this order, and said outside membrane portion is provided with a gradually changing shape in which an axial thickness from a top portion of said first fold-back portion to said outside root portion becomes thicker little by little, and the inside membrane portion arranged between said inner ring and the middle ring integrally has an inside root portion, a first fold-back portion protruding to the piston side, a straight portion, a second fold-back portion protruding to the clutch plate side and an outside root portion, between an inside fixed portion fixed to said inner ring and an outside fixed portion fixed to said middle portion, in this order, and said inside membrane portion is provided with a gradually changing shape in which an axial thickness from a top portion of said first fold-back portion to said inside root portion becomes thicker little by little. the clutch seal being an annular seal interposed between a clutch plate in a dry clutch and a piston pressing said clutch plate; a middle ring fixed to said piston between an outer ring and an inner ring which are fixed to a clutch housing; and membrane portions (15, 16) constructed by a rubber-like elastic body respectively between said outer ring and the middle ring and between said inner ring and the middle ring, characterized in that said middle ring is arranged at an intermediate position of a stroke in an initial shape of said membrane portion, the outside membrane portion arranged between said outer ring and the middle ring integrally has an outside root portion, a first fold-back portion protruding to the piston side, a straight portion, a second fold-back portion protruding to the clutch plate side and an inside root portion, between an outside fixed portion fixed to said outer ring and an inside fixed portion fixed to said middle ring, in this order, and said outside membrane portion is provided with a gradually changing shape in which an axial thickness from a top portion of said first fold-back portion to said outside root portion becomes thicker little by little, and the inside membrane portion arranged between said inner ring and the middle ring integrally has an inside root portion, a first fold-back portion protruding to the piston side, a straight portion, a second fold-back portion protruding to the clutch plate side and an outside root portion, between an inside fixed portion fixed to said inner ring and an outside fixed portion fixed to said middle portion, in this order, and said inside membrane portion is provided with a gradually changing shape in which an axial thickness from a top portion of said first fold-back portion to said inside root portion becomes thicker little by little.