A JNK inhibitor containing a compound having an isoquinolinone skeleton or a salt thereof, such as a compound represented by the formula ##STR00001## wherein ring A and ring B are each an optionally substituted benzene ring, X is --O--, --N.dbd., --NR.sup.3-- or --CHR.sup.3--, R.sup.2 is an acyl group, an optionally esterified or thioesterified carboxyl group, an optionally substituted carbamoyl group or an optionally substituted amino group and the like, a broken line shows a single bond or a double bond, and R.sup.1 is a hydrogen atom, an optionally substituted hydrocarbon group, an optionally substituted heterocyclic group and the like, and the like.