When an amount of visceral fat is measured using ultrasonic waves during medical examination of metabolic syndrome, three contact positions are defined on the surface of the abdominal region, and three distances inside the living body are measured by bringing a probe into contact with the surface of the abdominal region at the contact positions. Specifically, three measurement paths having a starting point at the center of the descending aorta are set, and the distances from the center to a surface of the fat layer adjacent to the surface of the body (inner surface of the subcutaneous layer) in the measurement paths are observed. An approximate area of a visceral fat region can be determined from the three distances and two angles defined by the three measurement paths, and an index value is calculated on the basis of the area. A probe holder having three holding portions is desirably used.