Provided is a method for purifying a quaternary alkyl ammonium salt enabling the obtainment of a particulate quaternary alkyl ammonium salt having a low water content at a high yield which is suitably used as an electrolyte in an electrolytic solution.The method for the purification of a quaternary alkyl ammonium salt comprises dispersing a quaternary alkyl ammonium salt to be purified into an organic solvent to allow it to form a suspension there. The alkyl groups constituting the quaternary alkyl ammonium salt have 1 to 5 carbon atoms each. The quaternary alkyl ammonium salt to be purified is dispersed into the organic solvent to make a suspension, and then the particles are washed with an alcohol solvent.