A production method of an ethylene and/or α-olefin-based ionomer, comprising: a heating conversion step of heating an ethylene and/or α-olefin / unsaturated carboxylic acid ester copolymer and converting the ester group in the copolymer into a metal-containing carboxylate containing a metal ion of Group 1, 2, or 12 of the periodic table, wherein the ethylene and/or α-olefin / unsaturated carboxylic acid ester copolymer is produced from at least one selected from the group consisting of ethylene and α-olefins having 3 to 20 carbon atoms and at least one or more unsaturated carboxylic acid esters, using a late transition metal catalyst, wherein (i) the heating conversion step comprises: converting the ethylene and/or α-olefin / unsaturated carboxylic acid ester copolymer into an ethylene and/or α-olefin / unsaturated carboxylic acid copolymer by hydrolysis or thermal decomposition; and reacting the ethylene and/or α-olefin / unsaturated carboxylic acid copolymer with a compound containing a metal ion of Group 1, 2, or 12 of the periodic table to convert the carboxylic acid in the ethylene and/or α-olefin / unsaturated carboxylic acid copolymer into the metal-containing carboxylate; or (ii) the heating conversion step comprises reacting with a compound containing a metal ion of Group 1, 2, or 12 of the periodic table while hydrolyzing or thermally decomposing the ester group of the ethylene and/or α-olefin / unsaturated carboxylic acid ester copolymer, to convert the ester group portion in the ethylene and/or α-olefin / unsaturated carboxylic acid ester copolymer into the metal-containing carboxylate; wherein the heating temperature in the heating conversion step is 100°C to 350°C; and wherein the ethylene and/or α-olefin / unsaturated carboxylic acid copolymer and the ethylene and/or α-olefin / unsaturated carboxylic acid ester copolymer have a melting point Tm observed by measurement using a differential scanning calorimeter and the total content Z in mol% of the unsaturated carboxylic acid unit and the unsaturated carboxylic acid ester unit included in the ethylene and/or α-olefin / unsaturated carboxylic acid copolymer and the ethylene and/or α-olefin / unsaturated carboxylic acid ester copolymer and the melting point Tm satisfy the following expression 50 < Tm < − 3.74 × Z + 130 a heating conversion step of heating an ethylene and/or α-olefin / unsaturated carboxylic acid ester copolymer and converting the ester group in the copolymer into a metal-containing carboxylate containing a metal ion of Group 1, 2, or 12 of the periodic table, wherein the ethylene and/or α-olefin / unsaturated carboxylic acid ester copolymer is produced from at least one selected from the group consisting of ethylene and α-olefins having 3 to 20 carbon atoms and at least one or more unsaturated carboxylic acid esters, using a late transition metal catalyst, wherein (i) the heating conversion step comprises: converting the ethylene and/or α-olefin / unsaturated carboxylic acid ester copolymer into an ethylene and/or α-olefin / unsaturated carboxylic acid copolymer by hydrolysis or thermal decomposition; and reacting the ethylene and/or α-olefin / unsaturated carboxylic acid copolymer with a compound containing a metal ion of Group 1, 2, or 12 of the periodic table to convert the carboxylic acid in the ethylene and/or α-olefin / unsaturated carboxylic acid copolymer into the metal-containing carboxylate; or (ii) the heating conversion step comprises reacting with a compound containing a metal ion of Group 1, 2, or 12 of the periodic table while hydrolyzing or thermally decomposing the ester group of the ethylene and/or α-olefin / unsaturated carboxylic acid ester copolymer, to convert the ester group portion in the ethylene and/or α-olefin / unsaturated carboxylic acid ester copolymer into the metal-containing carboxylate; (i) the heating conversion step comprises: converting the ethylene and/or α-olefin / unsaturated carboxylic acid ester copolymer into an ethylene and/or α-olefin / unsaturated carboxylic acid copolymer by hydrolysis or thermal decomposition; and reacting the ethylene and/or α-olefin / unsaturated carboxylic acid copolymer with a compound containing a metal ion of Group 1, 2, or 12 of the periodic table to convert the carboxylic acid in the ethylene and/or α-olefin / unsaturated carboxylic acid copolymer into the metal-containing carboxylate; or (ii) the heating conversion step comprises reacting with a compound containing a metal ion of Group 1, 2, or 12 of the periodic table while hydrolyzing or thermally decomposing the ester group of the ethylene and/or α-olefin / unsaturated carboxylic acid ester copolymer, to convert the ester group portion in the ethylene and/or α-olefin / unsaturated carboxylic acid ester copolymer into the metal-containing carboxylate; wherein the heating temperature in the heating conversion step is 100°C to 350°C; and wherein the ethylene and/or α-olefin / unsaturated carboxylic acid copolymer and the ethylene and/or α-olefin / unsaturated carboxylic acid ester copolymer have a melting point Tm observed by measurement using a differential scanning calorimeter and the total content Z in mol% of the unsaturated carboxylic acid unit and the unsaturated carboxylic acid ester unit included in the ethylene and/or α-olefin / unsaturated carboxylic acid copolymer and the ethylene and/or α-olefin / unsaturated carboxylic acid ester copolymer and the melting point Tm satisfy the following expression 50 < Tm < − 3.74 × Z + 130