To improve the body fat measuring precision of an image diagnostic device by taking very small unnecessary fat areas around the abdominal muscle and spinal cord of a patient into consideration by deciding the threshold range of the fat area of the patient based on the CT value of the fat area which varies at every patient.

In a tomographic image, the CT value or the like, of the fat area varies at every patient. Therefore, the threshold range of the fat areas peculiar to the image, namely, the patient is decided from the mean value and standard deviation of the pixel values of the fat areas by calculating the mean value and standard deviation of the pixel values of the fat areas from a region of interest on the surface of the patient and a region of interest in the viscus of the patient. In addition, in order to reduce the error of the body fat measurement, vary small fat areas scattered in the abdominal muscle and intestinal tract and around the backbone in the fat areas of the viscus are removed as unnecessary fat areas. Consequently, the body fat measuring precision of the image diagnosing device can be improved.