Pharmaceuticals which are effective for treatment, prevention, and the like of cancer and have less side effects are disclosed. The antitumor agent of the present invention comprises as an effective ingredient at least one leukotriene inhibitor. Examples of the leukotriene inhibitor includes leukotriene production inhibitors and leukotriene receptor antagonists, and preferred specific examples of the leukotriene inhibitor include montelukast, zafirlukast, pranlukast, and zileuton; pharmaceutically acceptable salts of these compounds; and pharmaceutically acceptable solvates of these compounds and the salts. The leukotriene inhibitor can also be used as a relieving agent for pain accompanying a tumor(s), and as a stromal hyperplasia inhibitor.