As a result of an effort made by us for the purpose of developing a therapeutic agent having a bone formation-stimulating effect by promoting the functions of osteoblasts, the present inventors discovered that a certain nitrogen-containing heterocyclic compound exhibits a potent bone formation-stimulating effect on the osteoblast and thus can serve as an excellent prophylactic or therapeutic agent against a metabolic bone disease, whereby establishing the present invention.Thus, the present invention provides a 3,6-disubstituted 1,2,4-triazolo[4,3-b]pyridazine compound or a pharmaceutically acceptable salt thereof as well as a pharmaceutical composition comprising such a compound and a pharmaceutically acceptable carrier, especially a bone-forming agent.