The present invention provides a benzofuran derivative of the formula [1]: ##STR00001## wherein x is a group of the formula: --N.dbd. or --CH.dbd.; Y is an optionally substituted amino group, an optionally substituted cycloalkyl group or an optionally substituted saturated heterocyclic group; A is a single bond, a carbon chain optionally having a double bond within or at the end(s) of the chain, or an oxygen atom; R.sup.1 is a hydrogen atom or a halogen atom; Ring B is an optionally substituted benzene ring; and R.sup.3 is a hydrogen atom, or a pharmaceutically acceptable salt thereof, which is useful as a medicament, especially as an activated blood coagulation factor X inhibitor.