To provide a method for performing a cross-coupling reaction of a Grignard compound with an alkyl halide simply, efficiently and in high yield, a method for obtaining a ω-bromo long chain carboxylic acid simply and efficiently using an easily obtainable raw material and a method for producing a useful branched fatty acid simply and efficiently [R1: an alkyl group having 1 to 15 carbon atoms, R2: an alkyl group having 1 to 30 carbon atoms with a carboxyl group and X and X′: a halogen atom] [n: an integer of 9 to 17] [n: an integer of 9 to 17, R1a: a branched alkyl group having 3 to 8 carbon atoms and X: a halogen atom].