A process for preparing a compound represented by general formulae (5) and (6) in the following reaction scheme or salts thereof, wherein R1 represents a protective group for a nitrogen atom; R2 represents a methanesulfonyl group or p-toluenesulfonyl group; R3 represents a hydrogen atom, an aralkyl group, or an alkyl group having 1 to 6 carbon atoms; and X represents a halogen atom. The above process is useful as an industrial process for preparing intermediates of anticoagulant aromatic amidine derivatives described in Japanese Patent Application Laid-Open (kokai) No. 208946/1993.