Antitumor agents with high antitumor effect have much side-effect, and its activity decreases when the toxicity decreases. Thus, satisfactory antitumor effects are not always obtained. Therefore, development of an antitumor agent having a high antitumor effect, with less side-effect and low toxicity is awaited. The present invention is to provide an antitumor agent of fine particles, wherein platinum chloride, chloroplatinic acids and hydroxyapatite are blend. Platinum chloride and chloroplatinic acids are platinum tetrachloride, tetrachloroplatinate, hexachloroplatinate, disodium platinum tetrachloride, disodium platinum hexachloride, dipotassium platinum tetrachloride, or dipotassium platinum hexachloride. The maximum particle diameter of hydroxyapatite is 1 .mu.m or less.