A quaternary ammonium hydroxycarboxylate is produced by quaternizing a tertiary amine with a hydroxycarboxylic ester represented by the following Formula 1: wherein R1 to R3 are as defined in the disclosure. By using only one tertiary amine having at least one substituent different from R1 of the ester moiety of the hydroxycarboxylic ester and by controlling the reaction conditions, a mixture of quaternary ammonium hydroxycarboxylates having different quaternary ammonium ions is produced. By reacting an inorganic acid with the quaternary ammonium hydroxycarboxylate or its mixture, a quaternary ammonium salt of inorganic acid or its mixture is produced.