There are disclosed a large conductance calcium-activated K channel opener comprising a compound of the formula (I): ##STR00001## wherein ring A is a 5-membered heterocyclic ring containing any one of O, N or S, which ring may be substituted by R.sup.4, R.sup.1 is aryl, heterocyclic or heterocycle-substituted carbonyl; R.sup.2 is hydrogen, halogen, carboxy, amino, alkyl, alkoxycarbonyl, alkenyl or cycloalkyl; R.sup.3 is aryl, heterocyclic or alkyl; and R.sup.4 is hydrogen or alkyl, each of substituents may be substituted, or a pharmaceutically acceptable salt thereof as an active ingredient.