Provided are: a device (1) and a system (100) for quantitatively calculating bone density by using an X-ray image of a bone pail, in particular, an alveolar bone； and an imaging assisting tool (4) for accurately and easily performing X-ray imaging of an alveolar bone. This bone density measurement device (1) comprises: an image display unit (101) for displaying X-ray images for an imaged bone part and a reference body on the same screen； a reference body density measurement unit (103) for measuring the density of the displayed X-ray image for the reference body； a bone part density measurement unit (105) for measuring the density of the displayed X-ray image for the bone part； and a bone density calculation unit (106) for calculating the hone density of the bone part by comparing the density of the X-ray image for the bone part with the density of the X-ray image for the reference body.
