The present invention has its object to provide an electrolyte anion which is high in decomposition temperature, in order to inhibit the electrolyte anion in the electrolyte solution for electrolytic capacitors from undergoing decarboxylation in the lead-free solder reflowing step to thereby prevent valve opening.The present invention uses an electrolyte solution comprising, as an electrolyte, the salt (A) composed of ammonium cation (a) and a polybasic carboxylic acid (b) anion, wherein the proton part charge of each carboxyl group in the polybasic carboxylic acid (b) as calculated by the quantum mechanics calculation software CAChe-based AM1 method is not higher than 0.243. Preferred is the polybasic carboxylic acid (b) represented by the general formula (1): wherein R1 to R4 may be the same or different and each represents a hydrogen atom, a functional group or a hydrocarbon group containing 1 to 3 carbon atoms, which may optionally contain a functional group, provided that at least one of R1 to R4 is an electron-donating group.