A 2-alkylthiobenzaldehyde of general formula (I) is reacted with a halo compound of general formula (II) to produce a 2-substituted benzo[b]thiophene of general formula (III): ##STR1## wherein R.sup.1 means an alkyl group of 1 to 4 carbon atoms,X.sup.1 CH.sub.2 Y (II)wherein X.sup.1 means Cl or Br; Y means --CO.sub.2 H, --CO.sub.2 R.sup.2, --COR.sup.2, --CONH.sub.2 or --CN; R.sup.2 means an alkyl group of 1 to 4 carbon atoms, ##STR2## wherein Y is as defined above. The process of this invention is of great economic and industrial value, for 2-acetylbenzo[b]thiophene and 2-benzo[b]thiophenecarboxylic acid, which are important intermediates for drugs and agricultural chemicals among others, can be obtained in a simple manner, easily and in high yield.