Résumé dans la langue de publication The present invnetion provides a remedy for erectile dysfunction. The active ingredient thereof is a fused pyridazine compound represented by the following formula (I) or a pharmacologically acceptable salt thereof: (wherein the ring C represents a 5 or 6 membered ring optionally having hetero atom(s); n is an integer of from 1 to 4; R**1 represents a hydrogen, a halogen, a cyano, etc.; A represents a hydrogen, a halogen, an optionally substituted amino, etc.; X represents a group represented by the formula -N=, etc.; and Y represents the formula -CO-, an optionally substituted amino, etc).