The present invention is a method of producing a 3-hydroxymethyl-benzo[b]thiophene derivative without the possible concomitant formation of isomers, which comprises selectively cyclizing a sulfoxide having X shown in the reaction scheme below: wherein R1 to R3 are, same or independently, a hydrogen atom, an alkyl group having 1 to 4 carbons, a trihalomethyl group, an alkoxy group having 1 to 4 carbons, an alkylthio group having 1 to 4 carbons, or a trihalomethoxy group; R4 is an acyl group; X is a halogen atom, a hydroxy group, an amino group, a mercapto group, an alkylthio group having 1 to 9 carbons, an acyloxy group having 1 to 9 carbons, an acylamino group having 1 to 9 carbons, or a trihalomethoxy group.