A pharmaceutical composition for topical administration for prevention and/or treatment of diseases associated with decrease in bone mass comprising an EP.sub.4 agonist as an active ingredient. An EP.sub.4 agonist, in which includes a compound possessing prostaglandin skeleton as a representative, possesses promoting action on bone formation, so it is useful for prevention and/or treatment of diseases associated with decrease in bone mass (bone diseases such as primary osteoporosis, secondary osteoporosis, bone metastasis of cancer, hypercalcemia, Paget's disease, bone loss and bone necrosis, postoperative osteogenesis, alternative therapy for bone grafting).