A compound represented by the following Formula (1) that is effective for the treatment of Paget's disease of bone or hypercalcemia or a medically acceptable solvate thereof; [wherein R1 refers to hydrogen atom, C1-C6 alkyl group optionally substituted with hydrogen group or C1-C6 alkoxy group optionally substituted with hydroxyl group, R2a and R2b refer to hydrogen atom, C1-C10 alkyl group optionally substituted with hydroxyl group, C6-C10 aryl group optionally substituted with hydroxyl group or C7-C12 aralkyl group optionally substituted with a hydroxyl group, or are combined to represent ethylene group. However, a compound in which R1 is a hydrogen atom or a methyl group and R2a and R2b are hydrogen atoms is excluded].