Provided is an electrolyte salt comprising a quaternary ammonium cation indicated by formula (1) and a trimethylsilyl alkanesulfonate anion indicated by formula (2). (In the formula, R1-R4 each independently indicate a C1-4 alkyl group or an alkoxyalkyl group indicated by —(CH2)n—OR. Any two among R1-R4 can mutually bond and form a ring together with a nitrogen atom to which same have bonded. The remaining two can mutually bond and form a spiro ring having a nitrogen atom as the spiro atom therefor. R indicates a methyl group or an ethyl group. n indicates 1 or 2 and m indicates 2 or 3.)