The purpose of the present invention is to provide an electrolyte solution which enables an electrochemical device to achieve a high capacitance retention rate even after long-time use at a high voltage. The present invention is an electrolyte solution which is characterized by containing a mononitrile compound (I), at least one compound (II) that is selected from the group consisting of dinitrile compounds and trinitrile compounds, and a quaternary ammonium salt (III), and which is also characterized in that the content of the compound (II) is 0.05-5% by mass in the electrolyte solution.