Novel platinum (II) complexes represented by general formula (I), wherein AB represents a bidentate diamine ligand, R1 and R2 independently represent a hydrogen atom or an alkyl group containing 1 to 8 carbon atoms, provided that R1 and R2 do not represent hydrogen atoms at the same time, Ar represents an aryl group, and n represents 0 or 1. The platinum (II) complexes have a strong growth inhibiting action on P-388 mouse leukemia cells and are used as novel substances having an oncostatic activity.