This invention is concerned with a ruthenium-iodo-optically active bidentate phosphine complex of the formula (I):[Ru--(I).sub.q --(T.sup.1).sub.n (SOL).sub.r (L)].sub.m (T.sup.2).sub.p (I).sub.s (I)wherein T.sup.1 represents a carboxylic acid anion, SOL represents a polar solvent, L represents an optically active bidentate phosphine ligand, T.sup.2 represents an anion different from halogen atom anions and carboxylic acid anions, n denotes 0 or 1, r denotes 0, 3 or 4, m denotes 1 or 2, q denotes 0 or 1, or where m is 2, q may represent 1 or 1.5, p denotes 0 or 1, and s denotes 0, 1 or 2 is prepared. Said phosphine complex is usefull as an efficient catalyst for asymmetrically hydrogenating 4-methylene-2-oxetanone into optically active 4-methyl-2-oxetanone.