Novel guanidinothiazole compounds of the general formula ##STR1## wherein R represents a hydrogen atom or a lower alkyl group, R.sub.1 represents an amino group, a lower alkyl group, a halogeno lower alkyl group, a substituted- or unsubstituted-aryl group, a mono- or di-lower alkylamino group, an arylamino group or an aralkylamino group, R.sub.2 represents a hydrogen atom, a lower alkyl group, a lower alkenyl group or a lower alkynyl group, Y represents a sulfur atom or a methylene group, m and n, each represents an integer of 1-3, and the pharmacologically acceptable acid addition salts thereof, they are useful as gastric acid secretion inhibitors.