PURPOSE: To obtain adipic acid in high reaction yield in high selectivity, by using inexpensive CH3CHO as a reaction accelerator, oxidizing cyclohexane in a specific amount of acetic acid solvent in the presence of a specified amount of a Co compound catalyst with feeding CH3CHO continuously.

CONSTITUTION: Cyclohexane is oxidized with a molecular oxygen-containing gas in a solvent of acetic acid in the presence of a Co compound (e.g., cobalt acetate, cobalt carbonate, etc.) and a reaction accelerator, to give adipic acid. In the reaction, acetaldehyde is used as the reaction accelerator, the amount of acetic acid used is 1W5 times, preferably 1.5W3 times the cyclohexane amount, the amount of the cobalt metal used is 0.1W1.0wt%, preferably 0.2W0.5wt% based on acetic acid, the amount of acetaldehyde used is 0.05W0.3 that of cyclohexane, and the oxidation reaction is carried out at a reaction temperature of 80W120°C at a reaction pressure of 5W30atm with feeding acetaldehyde continuously.