Disclosed is a method for the preparation of a .beta.-thiolactam compound, i.e. a 7-substituted-2-oxa-7-azabicyclo [3.2.0]-heptan-6-thione, represented by the general formula ##STR1## in which R is an alkyl or aryl group, having usefulness as an intermediate for the synthesis of various biologically active compounds. The compound can be prepared by the reaction of an isothiocyanate compound R--NCS, R being the same as above, and 2,3-dihydrofuran, preferably, under pressurization up to 2000 atmospheres or higher at an elevated temperature.