The present invention relates to a fused ring compound represented by the following formula [I]wherein each symbol is as defined in the specification, or a pharmaceutically acceptable a salt thereof, and a hepatitis C virus (HCV) polymerase inhibitor and a therapeutic agent for hepatitis C containing this compound. The compound of the present invention shows an anti-HCV activity based on the HCV polymerase inhibitory activity, and useful as an agent for the prophylaxis or treatment of hepatitis C.
