A method for producing an ionic polymer membrane comprising the following steps to: a step of converting a group represented by the following formula in a polymer (i) having a constituent unit having the group represented by the formula, to a group represented by the following formula to form a polymer (ii), a step of reacting the polymer (ii) with a compound represented by the following formula (a) to convert the group represented by the formula in the polymer (ii) to a group represented by the following formula to form a polymer (iii), a step of converting the group represented by the formula in the polymer (iii) to a group represented by the following formula to form a polymer (iv), a step of forming a polymer membrane which contains the polymer (i), prior to the step, or forming a polymer membrane which contains the polymer (ii), between the step and the step, or forming a polymer membrane which contains the polymer (iii), between the step and the step, or forming a polymer membrane which contains the polymer (iv), between the step and the following step, a step of thermally treating the polymer membrane containing the polymer (iv), subsequent to the step and between the step and the following step, a step of converting the group represented by the formula in the polymer (iv) to a group represented by the following formula to form a polymer (v), thereby to obtain an ionic polymer membrane which contains the polymer (v); -SO 2 F -SO 2 NZ 1 Z 2 FSO 2 (CF 2) 2 SO 2 F (a) -SO 2 N - (M α +)SO 2 (CF 2) 2 SO 2 F -SO 2 N - (H +)SO 2 (CF 2) 2 SO 2 F -SO 2 N - (M β +)SO 2 (CF 2) 2 SO 3 - M β + wherein each of Z 1 and Z 2 which are independent of each other, is a group selected from the group consisting of a hydrogen atom, a monovalent metal element and Si 3, R is a hydrogen atom, or a C 1-12 monovalent organic group which may have an etheric oxygen atom, and three R may be the same or different groups one another, M α + is a monovalent metal cation, or a monovalent cation derived from an organic amine, and M β + is an hydrogen ion, a monovalent metal cation, or a monovalent cation derived from an organic amine. a step of converting a group represented by the following formula in a polymer (i) having a constituent unit having the group represented by the formula, to a group represented by the following formula to form a polymer (ii), a step of reacting the polymer (ii) with a compound represented by the following formula (a) to convert the group represented by the formula in the polymer (ii) to a group represented by the following formula to form a polymer (iii), a step of converting the group represented by the formula in the polymer (iii) to a group represented by the following formula to form a polymer (iv), a step of forming a polymer membrane which contains the polymer (i), prior to the step, or forming a polymer membrane which contains the polymer (ii), between the step and the step, or forming a polymer membrane which contains the polymer (iii), between the step and the step, or forming a polymer membrane which contains the polymer (iv), between the step and the following step, a step of thermally treating the polymer membrane containing the polymer (iv), subsequent to the step and between the step and the following step, a step of converting the group represented by the formula in the polymer (iv) to a group represented by the following formula to form a polymer (v), thereby to obtain an ionic polymer membrane which contains the polymer (v); -SO 2 F -SO 2 NZ 1 Z 2 FSO 2 (CF 2) 2 SO 2 F (a) -SO 2 N - (M α +)SO 2 (CF 2) 2 SO 2 F -SO 2 N - (H +)SO 2 (CF 2) 2 SO 2 F -SO 2 N - (M β +)SO 2 (CF 2) 2 SO 3 - M β + wherein each of Z 1 and Z 2 which are independent of each other, is a group selected from the group consisting of a hydrogen atom, a monovalent metal element and Si 3, R is a hydrogen atom, or a C 1-12 monovalent organic group which may have an etheric oxygen atom, and three R may be the same or different groups one another, M α + is a monovalent metal cation, or a monovalent cation derived from an organic amine, and M β + is an hydrogen ion, a monovalent metal cation, or a monovalent cation derived from an organic amine. wherein each of Z 1 and Z 2 which are independent of each other, is a group selected from the group consisting of a hydrogen atom, a monovalent metal element and Si 3, R is a hydrogen atom, or a C 1-12 monovalent organic group which may have an etheric oxygen atom, and three R may be the same or different groups one another, M α + is a monovalent metal cation, or a monovalent cation derived from an organic amine, and M β + is an hydrogen ion, a monovalent metal cation, or a monovalent cation derived from an organic amine.