There are provided an aluminum electrolytic capacitor comprising an anode, a cathode comprising aluminum and an electrolytic solution containing an onium salt of fluorine-containing anion, wherein the electrolytic solution has a water concentration of 1% by weight or less, or wherein the cathode has a peak top of Al2p spectrum of 74.0 to 75.8 eV as measured by a method in which the surface of the cathode in the aluminum electrolytic capacitor heated at 125.degree. C. for 50 hours is analyzed by X-ray photoelectron spectroscopy (XPS); an electrolytic solution for electrolytic capacitor comprising a quaternary cyclic amidinium tetrafluoroaluminate and a solvent and containing specific impurity compounds in a total amount of 0.6% by weight or less, and an electrolytic capacitor using the electrolytic solution; and a method for preparing an organic onium tetrafluoroaluminate which is advantageously used in an aluminum electrolytic capacitor.