An aromatic nitrogen-containing 6-membered cyclic compound of the formula (I): wherein Ring A is a substituted or unsubstituted nitrogen-containing heterocyclic group; R1 is a substituted or unsubstituted lower alkyl group, —NH—Q—R3 (R3 is a substituted or unsubstituted nitrogen containing heterocyclic group, and Q is a lower alkylene group or a single bond), or —NH—R4 (R4 is a substituted or unsubstituted cycloalkyl group); R2 is a substituted or unsubstituted aryl group; one of Y and Z is ═CH—, and the other is ═N—, or a pharmaceutically acceptable salt thereof, these compounds exhibiting excellent selective PDE V inhibitory activities, and hence, being useful in the prophylaxis or treatment of penile erectile dysfunction, etc.