The present invention provides a fused ring compound of the following
formula [I] ##STR00001## wherein each symbol is as defined in the
specification, a pharmaceutically acceptable salt thereof, and a
therapeutic agent for hepatitis C, which contains this compound. The
compound of the present invention shows an anti-hepatitis C virus (HCV)
action based on the HCV polymerase inhibitory activity, and is useful as
a therapeutic agent or prophylactic agent for hepatitis C.