Compounds expressed by the following formula (1) or pharmaceutically permissible solvate thereof which is effective for treating Paget's disease of bone, hypercalcemia or osteoporosis. In the formula, R1 is a C2-C10 alkyl group, or a C7-C15 aralkyl group whose aromatic ring is optionally substituted with a C1-C6 alkyl group, a C1-C6 alkoxyl group, a hydroxyl group, a halogen atom or a trifluoromethyl group; and R2 is a C1-C10 alkyl group, or a C7-C15 aralkyl group whose aromatic ring is optionally substituted with a C1-C6 alkyl group, a C1-C6 alkoxyl group, a hydroxyl group, a halogen atom or a trifluoromethyl group.