A novel and simple method is proposed for the synthetic preparation of an N,N-disubstituted .beta.-ketothioamide compound represented by the general formula R.sup.1 R.sup.2 N--CS--CH.sub.2 --CO--R.sup.3, in which R.sup.1 and R.sup.2 are each a monovalent hydrocarbon group or each a divalent hydrocarbon group jointly forming a cyclic structure together with the nitrogen atom and R.sup.3 is a hydrogen atom, a monovalent hydrocarbon group or a divalent hydrocarbon group forming a cyclic structure jointly with R.sup.1, R.sup.1 being a divalent hydrocarbon group and R.sup.2 being a monovalent hydrocarbon group. The method comprises:(a) mixing an N,N-disubstituted amide compound represented by the general formula R.sup.1 R.sup.2 N--CO--R.sup.3, and a bis(trialkylsilyl)thioketene compound represented by the general formula (R.sub.3 Si).sub.2 C.dbd.C.dbd.S, in which each R is an alkyl group, to effect an addition reaction therebetween forming an intermediate compound; and (b) dissolving the intermediate compound obtained in step (a) in an alcohol.