To avoid disadvantageous adverse reactions such as gel formation and excess foaming and obtain a tertiary amine oxide useful as a component for composition such as cleaning agent in a good yield.

An amine oxide compound of formula I in an amount of 1-15wt.% based on the reaction mixture is previously added thereto, when the amine oxide compound of formula I is obtained by reaction of a tertiary amine of the formula R1R2R3N (R1 is a 1-3C alkyl; R2 is a 1-20C alkyl; R3 is a 6-22C alkyl) with a compound capable of separating oxygen, e.g. hydrogen peroxide aqueous solution.