To provide a pharmaceutical agent or an antitumor agent useful for the treatment and/or prevention of gastrointestinal cancer, leukemia, pituitary tumor, small cell lung cancer, thyroid cancer, and neuroastrocytoma. The antitumor agent containing, as an active ingredient, a 1,5-benzodiazepine derivative represented by the following formula (1) : (wherein R 1 represents a C 1-6 alkyl group; R 2 represents a phenyl group or a cyclohexyl group; and Y represents a single bond or a C 1-4 alkylene group) or a pharmaceutically acceptable salt thereof.