The present invention relates to a pharmaceutical composition for preventing or treating osteoporosis which comprises neuropeptide Y as an active ingredient. The neuropeptide Y according to the present invention reduces the expression of a cell adhesion factor in osteoblasts in which a Y1 receptor is present, and consequently an effect of releasing a haematopoietic stem cell from bone marrow into the blood is excellent. When a haematopoietic stem cell is released into the blood, the number of osteoclasts which induce an osteoporotic lesion by differentiation from the haematopoietic stem cell decreases, and thus progression of a bone erosion caused by osteoclasts can be prevented. Accordingly, the neuropeptide Y which is an active ingredient of the composition of the present invention is useful as a therapeutic agent for osteoporosis.