To obtain a substance capable of preventing induction of breast cancer.

This breast cancer suppressant comprises melatonin or melatonin analogue. The breast cancer suppressant is daily administered for a definite period to human male to prevent induction of breast cancer. The administration may be carried out in combination of the melatonin or melatonin analogue and progestogen and/or oestrogen. Daily dose of melatonin is 2 mg to 1,000/70 kg weight female and daily dose of progestogen is 7.5 μg to 2,500 μg and daily dose of oestrogen is 5 μg to 100 μg. The administration is carried out by vein injection or subcutaneous injection in orally or physiologically proper carrier.