To obtain a compound having large antitumor activities against various kinds of cancer, i.e., anti-malignant tumor activities, capable of inhibiting the proliferation of fibroblast, and having small toxicity, its salt and a derivative thereof.

This diphenylmethylpiperazine derivative is an anticancer agent characterized by a compound represented by general formula [I] (wherein, R is a group of general formula [II] or general formula [III]) or its salt, or a derivative or a prodrug thereof. The derivative has the toxicity smaller than that of a conventional anticancer agent, and having excellent anticancer activities against various kinds of solid carcinoma. The derivative is also effective against pulmonary fibrosis disease and proliferative lesion of keloid by inhibiting the proliferation of the fibroblast.