Disclosed is a ruthenium-phosphine complex represented by formula (I):�{RuX(L)}.sub.2 (.mu.-X).sub.3 !.sup.- �R.sub.2 NH.sub.2 !.sup.+(I)wherein R is hydrogen, an alkyl group having 1 to 5 carbon atoms, a cycloalkyl group, a substituted or unsubstituted phenyl group or a substituted or unsubstituted benzyl group; X is halogen; and L is an optically active or inactive tertiary phosphine. A process for preparing the complex (I) is also disclosed, which comprises (A) reacting a ruthenium complex represented by formula: �RuX(arene)(L)!X (wherein arene is a substituted or unsubstituted phenyl group) and an ammonium salt represented by formula (III): R.sub.2 NH.HX, or (B) reacting a ruthenium complex represented by formula: �RuX.sub.2 (arene)!.sub.2, a tertiary phosphine represented by L, and an ammonium salt (III). The complex (I) useful as a catalyst for general syntheses or asymmetric syntheses can be obtained easily in good yield.