An aromatic compound (C) is reacted with carbon monoxide (D) and molecular oxygen (E) in the presence of a palladium compound catalyst (A) and a catalyst (B) and thereby yields a aromatic carboxylic acid corresponding to the aromatic compound (C) except with one or more carboxyl groups bonded to its aromatic ring. The catalyst (B) contains a heteropolyacid or a salt thereof (B1) or a mixture of oxo acids and/or salts thereof (B2), and the mixture (B2) contains, as a whole, one of P and Si and at least one selected from V, Mo and W.