A formylated alkylbenzenesulfonyl halide compound represented by the following general formula (I): ##STR1## wherein R.sub.1 is an alkyl group having 1 to 5 carbon atoms; R.sub.2 and R.sub.3 are each a member selected from the group consisting of a hydrogen atom and alkyl groups having 1 to 5 carbon atoms; X is a halogen atom; the sulfonyl halide group is present at the ortho or para position while the formyl group at the para or ortho position relative to the group R.sub.1 when R.sub.2 and R.sub.3 are both a hydrogen atom, while the sulfonyl halide group and the formyl group are present at the para positions relative to the groups R.sub.1 and R.sub.2, respectively, when R.sub.2 is an alkyl group and R.sub.3 is a hydrogen atom, or the sulfonylhalide group and the formyl group are present at the para positions relative to the groups R.sub.1 and R.sub.2, respectively, when R.sub.2 and R.sub.3 are both an alkyl group; the compound (I) being prepared by reacting an alkylbenzene represented by the following formula (III) with carbon monoxide and a halosulfonic acid represented by the following formula (II) in the presence of an antimony halide:XSO.sub.3 H (II) ##STR2## wherein all of X in the formula (II) and R.sub.1, R.sub.2 and R.sub.3 in the formula (III) are as defined above.