PURPOSE: To reduce power cost while increasing conversion efficiency and to obtain aromatic carboxylic acid of high quality by oxidizing aromatic alkyl in a liquid phase by using an oxidizing reactor enhanced in mixing characteristics.

CONSTITUTION: A stirrer 12 and the shaft 15 connected thereto are provided in a continuous stirring reaction tank 10 composed of a pressure-resistant container and an upper mixing element 13 composed of a disc turbine fitted with six blades and a lower mixing element 14 composed of a 6-blade turbine fitted with a pitch moving a liquid upwardly and outwardly from the shaft at a time of rotation are fixed to the shaft and a relatively narrow vertically arranged inner buffle 16 is provided. A mixture of a volatile aq. acid soln. medium containing aromatic alkyl and a catalyst is introduced into the reaction tank 10 from a conduit 25 and oxygen-containing gas is introduced into the reaction tank 10 from a gas introducing pipe 18 and a reaction product is taken out of a conduit 26. Heat generated by reaction is removed from the reaction mixture by the evaporation of the aq. solvent and a small amt. of aromatic alkyl and the evaporated substances are sent to a tower top condenser 20 and the condensed part is refluxed to the reaction tank 10.