A bone density measuring device, comprising: an image display unit for displaying an X-ray image relating to a photographed bone portion of a targeted tooth of a person and a plurality of reference bodies, the bone density values of which are given, on the same screen, the X-ray image being an image captured with an imaging aid including a holding portion for holding a planar imaging plate capturing the X-ray image, the imaging plate being substantially orthogonal to the imaging main axis of the main body of the imaging aid, wherein at one end of the holding portion an elongated rod-shaped support portion is provided for supporting the holding portion at an appropriate position when the support portion is engaged by the person to be imaged and fixed so as to be directed towards the tooth root direction and further wherein a case accommodating the reference bodies (3a, 3b, 3c) is set at the end portion of the support portion on the holding portion side so that the reference bodies (3a, 3b, 3c) can be detected by the imaging plate together with the alveolar bone portion of the targeted tooth; a reference body image density measuring unit configured to measure the density of the X-ray image of the displayed plurality of reference bodies; a bone portion image density measurement unit configured to measure the density of the X-ray image of the displayed bone portion; a bone density calculation unit configured to calculate the bone density of the bone portion by comparing the X-ray image density of the bone portion with the X-ray image density of the reference bodies, wherein said bone density calculation unit is configured to calculate bone density of the bone portion by (a) determining a relation between an X-ray image density of the plurality of reference bodies and the corresponding bone density value, (b) approximating the determined relation to obtain a relation expressed by a linear regression line between the X-ray image densities of the reference bodies and the corresponding bone density values, and (c) using said linear regression line as a reference to calculate the bone density of the bone portion based on the measured X-ray image density of the bone portion. an image display unit for displaying an X-ray image relating to a photographed bone portion of a targeted tooth of a person and a plurality of reference bodies, the bone density values of which are given, on the same screen, the X-ray image being an image captured with an imaging aid including a holding portion for holding a planar imaging plate capturing the X-ray image, the imaging plate being substantially orthogonal to the imaging main axis of the main body of the imaging aid, wherein at one end of the holding portion an elongated rod-shaped support portion is provided for supporting the holding portion at an appropriate position when the support portion is engaged by the person to be imaged and fixed so as to be directed towards the tooth root direction and further wherein a case accommodating the reference bodies (3a, 3b, 3c) is set at the end portion of the support portion on the holding portion side so that the reference bodies (3a, 3b, 3c) can be detected by the imaging plate together with the alveolar bone portion of the targeted tooth; a reference body image density measuring unit configured to measure the density of the X-ray image of the displayed plurality of reference bodies; a bone portion image density measurement unit configured to measure the density of the X-ray image of the displayed bone portion; a bone density calculation unit configured to calculate the bone density of the bone portion by comparing the X-ray image density of the bone portion with the X-ray image density of the reference bodies, wherein said bone density calculation unit is configured to calculate bone density of the bone portion by (a) determining a relation between an X-ray image density of the plurality of reference bodies and the corresponding bone density value, (b) approximating the determined relation to obtain a relation expressed by a linear regression line between the X-ray image densities of the reference bodies and the corresponding bone density values, and (c) using said linear regression line as a reference to calculate the bone density of the bone portion based on the measured X-ray image density of the bone portion.