An alkaline water electrolysis diaphragm comprising: a sheet-shaped porous supporting body; and a porous membrane containing organic polymer resin and obtainable by impregnating in the supporting body from one surface (α) of the supporting body to the opposite surface (β) of the supporting body, wherein: a thickness of the porous membrane is larger than a thickness of the supporting body; when one of the membrane surfaces of the porous membrane is defined as a surface A, the other membrane surface of the porous membrane opposite to the surface A is defined as a surface B, a section of the porous membrane parallel to the surfaces A and B is defined as a section C, an average pore diameter of the porous membrane on the surface A is defined as an average pore diameter D A, an average pore diameter of the porous membrane on the surface B is defined as an average pore diameter D B, and an average pore diameter of the porous membrane on the section C is defined as an average pore diameter D C, a ratio of the average pore diameter D B to the average pore diameter D A is 0.9 to 1.1, the section C having the average pore diameter D C that is larger than each of the average pore diameters D A and D B is included in the porous membrane; and when a section or surface parallel to the surfaces A and B of the porous membrane is defined as a surface S, and an average pore diameter of the porous membrane on the surface S is defined as an average pore diameter D S, the surface S having the average pore diameter D S that is not less than the average pore diameter D A or D B and smaller than 2μm is included in an impregnated region that is a region of the porous membrane and is impregnated in the supporting body. a sheet-shaped porous supporting body; and a porous membrane containing organic polymer resin and obtainable by impregnating in the supporting body from one surface (α) of the supporting body to the opposite surface (β) of the supporting body, wherein: a thickness of the porous membrane is larger than a thickness of the supporting body; when one of the membrane surfaces of the porous membrane is defined as a surface A, the other membrane surface of the porous membrane opposite to the surface A is defined as a surface B, a section of the porous membrane parallel to the surfaces A and B is defined as a section C, an average pore diameter of the porous membrane on the surface A is defined as an average pore diameter D A, an average pore diameter of the porous membrane on the surface B is defined as an average pore diameter D B, and an average pore diameter of the porous membrane on the section C is defined as an average pore diameter D C, a ratio of the average pore diameter D B to the average pore diameter D A is 0.9 to 1.1, the section C having the average pore diameter D C that is larger than each of the average pore diameters D A and D B is included in the porous membrane; and when a section or surface parallel to the surfaces A and B of the porous membrane is defined as a surface S, and an average pore diameter of the porous membrane on the surface S is defined as an average pore diameter D S, the surface S having the average pore diameter D S that is not less than the average pore diameter D A or D B and smaller than 2μm is included in an impregnated region that is a region of the porous membrane and is impregnated in the supporting body. a thickness of the porous membrane is larger than a thickness of the supporting body; when one of the membrane surfaces of the porous membrane is defined as a surface A, the other membrane surface of the porous membrane opposite to the surface A is defined as a surface B, a section of the porous membrane parallel to the surfaces A and B is defined as a section C, an average pore diameter of the porous membrane on the surface A is defined as an average pore diameter D A, an average pore diameter of the porous membrane on the surface B is defined as an average pore diameter D B, and an average pore diameter of the porous membrane on the section C is defined as an average pore diameter D C, a ratio of the average pore diameter D B to the average pore diameter D A is 0.9 to 1.1, the section C having the average pore diameter D C that is larger than each of the average pore diameters D A and D B is included in the porous membrane; and when a section or surface parallel to the surfaces A and B of the porous membrane is defined as a surface S, and an average pore diameter of the porous membrane on the surface S is defined as an average pore diameter D S, the surface S having the average pore diameter D S that is not less than the average pore diameter D A or D B and smaller than 2μm is included in an impregnated region that is a region of the porous membrane and is impregnated in the supporting body.