PURPOSE: To obtain an inexpensive aromatase inhibitor useful for preventing and treating estrogen-dependent diseases such as mammary cancer, etc., containing a naphthoquinone compound.

CONSTITUTION: A 1,4-naphthoquinone or a 1,2-naphthoquinone, a derivative thereof shown by formula I and/or formula II (R1 and R2 are H or OH; R3 is H or 1W6C hydrocarbon chain) as an active ingredient is blended with a carrier, excipient, etc., and pharmaceutically manufactured by a conventional procedure to give the aimed substance. The inhibitor can be administrated by various methods such as oral, hypodermic, intramuscular, percutaneous or rectal administration. The blend can be pharmaceutically manufactured in such a way that the active ingredient is released rapidly, continuously or in a delayed way.