A derivative of thiazolo [3,2-a] pyrimidine expressed by formula (I) ##STR1## (where R indicates a phenyl group or a benzyl group which has a halogen atom, a lower alkyl or lower alkyloxy group as a substituent group, an alicyclic group or an arylethyl group), and a process for the preparation of a compound of formula (I) comprising cyclizing a compound expressed by formula (II) by application of heat ##STR2## (where a definition of R is as same as that given in case of formula (I) and R' indicates a halogen atom or lower alkyloxy group). A drug which contains this compound as active ingredient is useful for curing autoimmune diseases such as rheumatoid arthritis, nephritis, etc.