Carbostyril compounds represented by the formula (I): ##STR1## wherein R.sub.1 represents a hydrogen atom or an alkyl group; R.sub.2 represents a hydrogen atom, an alkyl group or a phenylalkyl group; R.sub.3 represents a hydrogen atom and R.sub.4 represents a phenylalkyl group or a phenoxyalkyl group; the 3,4-bond of the carbostyril nucleus represents a single or double bond, pharmaceutically acceptable acid addition salts thereof, and process for preparing the same.