A URAT1 activity inhibitor containing a nitrogen-containing fused ring compound represented by the following formula [1]: wherein each symbol is as defined in the description. The present invention is useful for the prophylaxis or treatment of pathology showing involvement of uric acid, such as hyperuricemia, gouty tophus, acute gouty arthritis, chronic gouty arthritis, gouty kidney, urolithiasis, renal function disorder, coronary artery disease, ischemic heart disease and the like.