A method for producing an alkali metal salt, comprising: a pretreatment step of removing a purification-inhibiting substance from an aqueous alkali metal salt solution with a separation membrane selected to have a glucose removal ratio and an isopropyl alcohol removal ratio simultaneously satisfying the following expressions and when each of a 1000 ppm aqueous glucose solution having a temperature of 25°C and pH of 6.5 and a 1000 ppm aqueous isopropyl alcohol solution having a temperature of 25°C and pH of 6.5 is permeated through the separation membrane at an operating pressure of 0.75 MPa and a recovery step of recovering an alkali metal salt from the aqueous alkali metal salt solution after the pretreatment step: 90 % > Glucose removal ratio ≥ 50 % Glucose removal ratio − Isopropyl alcohol removal ratio ≥ 50% wherein the purification-inhibiting substance includes alkaline earth metal salts and sulfate salts, and wherein the separation membrane is a nanofiltration membrane having a nonwoven fabric substrate, a polysulfone support provided on the substrate, and a polyamide functional layer provided on the support, wherein the polyamide functional layer contains a crosslinked polyamide obtained by interfacial polymerization of a piperazine compound represented by the general formula with a divalent or multivalent polyfunctional acid halide, wherein R 1 of the general formula represents an alkyl group having 1 to 6 carbon atoms, a phenyl group, a benzyl group, COOR 3, CONHR 3, CONR 3 2 or OR 3, and R 3 represents a hydrogen, an alkyl group having 1 to 6 carbon atoms, a phenyl group or a benzyl group, and R 2 represents a hydrogen, an alkyl group having 1 to 6 carbon atoms, a phenyl group, a benzyl group, COOR 4, CONHR 4, CONR 4 2 or OR 4, and R 4 represents a hydrogen, an alkyl group having 1 to 6 carbon atoms, a phenyl group or a benzyl group. a pretreatment step of removing a purification-inhibiting substance from an aqueous alkali metal salt solution with a separation membrane selected to have a glucose removal ratio and an isopropyl alcohol removal ratio simultaneously satisfying the following expressions and when each of a 1000 ppm aqueous glucose solution having a temperature of 25°C and pH of 6.5 and a 1000 ppm aqueous isopropyl alcohol solution having a temperature of 25°C and pH of 6.5 is permeated through the separation membrane at an operating pressure of 0.75 MPa and a recovery step of recovering an alkali metal salt from the aqueous alkali metal salt solution after the pretreatment step: 90 % > Glucose removal ratio ≥ 50 % Glucose removal ratio − Isopropyl alcohol removal ratio ≥ 50% wherein the purification-inhibiting substance includes alkaline earth metal salts and sulfate salts, and wherein the separation membrane is a nanofiltration membrane having a nonwoven fabric substrate, a polysulfone support provided on the substrate, and a polyamide functional layer provided on the support, wherein the polyamide functional layer contains a crosslinked polyamide obtained by interfacial polymerization of a piperazine compound represented by the general formula with a divalent or multivalent polyfunctional acid halide, wherein R 1 of the general formula represents an alkyl group having 1 to 6 carbon atoms, a phenyl group, a benzyl group, COOR 3, CONHR 3, CONR 3 2 or OR 3, and R 3 represents a hydrogen, an alkyl group having 1 to 6 carbon atoms, a phenyl group or a benzyl group, and R 2 represents a hydrogen, an alkyl group having 1 to 6 carbon atoms, a phenyl group, a benzyl group, COOR 4, CONHR 4, CONR 4 2 or OR 4, and R 4 represents a hydrogen, an alkyl group having 1 to 6 carbon atoms, a phenyl group or a benzyl group.