A diaphragm includes a porous supporting body and a polymer porous membrane. When one of surfaces of the porous membrane is defined as a surface A, the other surface 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 on the surface A is defined as an average pore diameter DA, an average pore diameter on the surface B is defined as an average pore diameter DB, and an average pore diameter on the section C is defined as an average pore diameter DC, the average pore diameters DA and DB are substantially equal to each other, and the average pore diameter DC is larger than each of the average pore diameters DA and DB.