An object of the present invention is to provide pharmaceutical agents that reduce risk for decrease in bone mineral density due to their effect of reducing estrogen levels and exert excellent therapeutic effects on endometriosis. The present invention relates to pharmaceutical compositions for treating endometriosis comprising 3-[2-fluoro-5-(2,3-difluoro-6-methoxybenzyloxy)-4-methoxyphenyl]-2,4-dioxo-1,2,3,4-tetrahydrothieno[3,4-d]pyrimidine-5-carboxylic acid or a pharmaceutically acceptable salt thereof, which are administered orally once a day at a daily dose of between 50 mg and 75 mg calculated as a free form.
