An anticoagulant agent which comprises, as an active ingredient, an aromatic amidine derivative represented by the following general formula (1) or a salt thereof: ##STR1## wherein the group represented by ##STR2## is a group selected from indolyl, benzofuranyl, benzothienyl, benzimidazolyl, benzoxazolyl, benzothiazolyl, naphthyl, tetrahydronaphthyl and indanyl; X is a single bond, an oxygen atom, a sulfur atom or a carbonyl group; and Y is a saturated or unsaturated 5- or 6-membered heterocyclic moiety or cyclic hydrocarbon moiety optionally having a substituent group, an amino group optionally having a substituent group or an aminoalkyl group optionally having a substituent group.The inventive compound has a high anticoagulant capacity based on its excellent FXa inhibition activity.