This invention provides an indenothiazole derivative and a process for manufacturing the same. The said indenothiazole derivative is represented by the following formula: ##STR1## wherein X and Y are identical with, or different from, each other and are each a hydrogen atom, halogen atom, low alkyl group or low alkoxy group; n is an integer of from 0 to 4; R.sub.1 is a hydrogen atom, low alkyl group, unsubstituted or substituted phenyl group; R.sub.2 is a low alkyl group, cycloalkyl group, low alkoxyl group, unsubstituted or substituted phenoxy group, unsubstituted or substituted phenyl group, alkenyl group, heterocyclic group or cycloamino group; R.sub.3 is a hydrogen atom, low alkyl group or acyl group; and R.sub.2 and R.sub.3 may be combined to form a cycloamino group. Further, this invention also provides a tautomer of the compound represented by the above formula (Ia), wherein all the symbols are as defined above except that R.sub.3 is limited to a hydrogen atom.