Disclosed is a method for producing a lithium ion battery electrolyte solution containing lithium hexafluorophosphate as an electrolyte and a lithium ion battery using the electrolyte solution. The electrolyte solution is produced by reacting lithium chloride with phosphorus trichloride and chlorine in a non-aqueous organic solvent, reacting a reaction product generated in the solvent with hydrogen fluoride, reacting unreacted remaining hydrogen fluoride with lithium chloride, and then, separating the resulting reaction solution by filtration into a filtrate and a solid residue. The filtrate is obtained as the lithium ion battery electrolyte solution. The solid product is further reacted with phosphorous trichloride and chlorine in a non-aqueous organic solvent. The reaction product generated in the solvent is reacted with hydrogen fluoride, followed by reacting unreacted remaining hydrogen fluoride with lithium chloride. It is possible to accomplish reduction in production cost with the reuse of the reaction residue as production raw material.