To obtain a carboxylic acid or its salt useful as a surfactant, etc., by oxidizing a primary hydroxyl group-having compound with a specific oxidizing agent in the presence of a specified amine in a high yield and in a good selectivity.

This method for producing a carboxylic acid or its salt comprises oxidizing a primary hydroxyl group-having compound such as a 8-18C aliphatic alcohol, a polyoxyalkylenealkylether, an alkylamine-ethylene oxide adduct or an aliphatic alcohol with 1.5-5.0 moles of a halogen-based oxidizing agent such as chlorine or a hypochlorite in the presence of 0.005-0.2 mole of a hindered amine compound such as tetramethylpiperidine. The reaction is preferably carried out by controlling the pH of the reaction system to 8.0-9.0 with a pH-controlling agent such as formic acid or acetic acid at 0-30°C and further aging the reaction at 20-60°C. In order to moderate the advance of the reaction, a solvent such as paraffin or ethylene glycol dimethyl ether is used, and the reaction is, if necessary, subjected to the irradiation of UV light, the bubbling of air, etc.