A new platinum complex which has strong antitumor activity and has effect with smaller doses and a pharmaceutical composition containing the same are provided. A spiro[4,4]nonane-1,6-diamineplatinum(II) complex which is represented by a following general formula (A). (In the formula, X and Y are same or different, and X and Y represent halogen atoms respectively, or X and Y cooperatively represent a divalent residue which is described by a formula (Z).) The platinum complex has a strong antitumor activity and is efficacious as a therapeutic agent for malignant tumors.