A catalytic amount of magnesium bromide diethyl etherate (MgBr2.Et2O) enables us to carry out the aerobic photo-oxidation of a methyl group at the aromatic nucleus to the corresponding carboxylic acid in high yield under irradiation of VIS from a general-purpose fluorescent lamp. The bromine radical is thought to be generated in situ by continuous aerobic photo-oxidation of the bromine anion from MgBr2.Et2O, and to effect this oxidation reaction.