To obtain a high-purity halogenated quaternary ammonium compound useful as a raw material for a phase-transfer catalyst, an antimicrobial agent and an agent for electronic industries, by reacting a tertiary amine with a halogenated alkyl in the presence of a basic quaternary ammonium compound.

An equivalent or optionally a considerably excessive amount of a halogenated alkyl is reacted with a tertiary amine in the presence of 0.01-10 mol% based on the tertiary amine of a basic quaternary ammonium compound (preferably quaternary ammonium hydroxide compound) at 0-120°C under 0-5 kg/cm2 to give the objective compound containing amine salts as impurities as low as possible. The minimum amount of the basic quaternary ammonium compound to suppress the emission of a hydrohalide prepared as a by-product in the reaction of the tertiary amine and the halogenated alkyl is measured. The above-mentioned reaction is carried out in the presence of 0-500 mol% of the minimum amount of the basic quaternary ammonium and the unreacted ammonium compound is neutralized to give the objective compound.