Provided are: an electrolyte solution having high electrolyte salt concentration, which is not susceptible to decrease in the cycle characteristics and is capable of improving service life performance of a battery; and a lithium ion secondary battery which uses this electrolyte solution. An electrolyte solution of the present invention contains an electrolyte salt and a solvent. The concentration of the electrolyte salt is more than 1.1 mol/L, and a compound represented by general formula (1) is contained as the electrolyte salt. A cyclic carbonate is contained as the solvent. (XSO2)(FSO2)NLi (1) (In general formula (1), X represents a fluorine atom, an alkyl group having 1-6 carbon atoms or a fluoroalkyl group having 1-6 carbon atoms.)