A gasket for holding a diaphragm in an alkaline water electrolyser, said gasket having a square shape or an annular shape, comprising: first and second surfaces (13a, 13b) configured to respectively contact a frame near a positive electrode and a frame near a negative electrode; a slit portion which is opened inward and configured to accommodate the edge of the diaphragm; a filling portion which is provided outside the slit portion and not configured to accommodate the diaphragm, first and second portions (50, 52) which face each other with the slit portion interposed therebetween in the facing direction between the first surface and the second surface, the first portion including the first surface and the second portion including the second surface; and four divided portions (20a - 20d) which are provided in the first portion and/or the second portion each being inclined with respect to the first and/or second surface (13a, 13b) and configured to be opened inward to the inside of the first and/or second portion (50, 52) to divide the first portion and/or second portion so that the first surface is continuous to the surface defining the slit portion in the first portion and/or the second surface is continuous to the surface defining the slit portion in the second portion, wherein the first portion is configured to be turned up from the divided portion (20a - 20d) as a start point so as to be separated from the second portion in the gasket, wherein the slit portion and the filling portion are provided in the entire periphery of the gasket, wherein the first and second surfaces (13a, 13b) are formed as flat surfaces, wherein the slit portion extends so as to be substantially parallel to the first and second surfaces (13a, 13b), wherein the divided portions (20a, 20b, 20c, 20d) are respectively provided at corner portions (C1- C4) of the gasket, wherein each of the four corner portions (C1-C4) is an intersecting portion of an inner surface of the first portion and/or the second portion, wherein the divided portions (20a - 20d) have a same configuration, each of the divided portions (20a - 20d) dividing a part of the first portion having an annular shape, and wherein each of the divided portions (20a, 20b, 20c, 20d) has surfaces and, and the gap between the surface and the surface 20ab is substantially equal to the slit portion or smaller than the slit portion. first and second surfaces (13a, 13b) configured to respectively contact a frame near a positive electrode and a frame near a negative electrode; a slit portion which is opened inward and configured to accommodate the edge of the diaphragm; a filling portion which is provided outside the slit portion and not configured to accommodate the diaphragm, first and second portions (50, 52) which face each other with the slit portion interposed therebetween in the facing direction between the first surface and the second surface, the first portion including the first surface and the second portion including the second surface; and four divided portions (20a - 20d) which are provided in the first portion and/or the second portion each being inclined with respect to the first and/or second surface (13a, 13b) and configured to be opened inward to the inside of the first and/or second portion (50, 52) to divide the first portion and/or second portion so that the first surface is continuous to the surface defining the slit portion in the first portion and/or the second surface is continuous to the surface defining the slit portion in the second portion, wherein the first portion is configured to be turned up from the divided portion (20a - 20d) as a start point so as to be separated from the second portion in the gasket, wherein the slit portion and the filling portion are provided in the entire periphery of the gasket, wherein the first and second surfaces (13a, 13b) are formed as flat surfaces, wherein the slit portion extends so as to be substantially parallel to the first and second surfaces (13a, 13b), wherein the divided portions (20a, 20b, 20c, 20d) are respectively provided at corner portions (C1- C4) of the gasket, wherein each of the four corner portions (C1-C4) is an intersecting portion of an inner surface of the first portion and/or the second portion, wherein the divided portions (20a - 20d) have a same configuration, each of the divided portions (20a - 20d) dividing a part of the first portion having an annular shape, and wherein each of the divided portions (20a, 20b, 20c, 20d) has surfaces and, and the gap between the surface and the surface 20ab is substantially equal to the slit portion or smaller than the slit portion.