A diagnostic index that indicates a condition of obesity of an abdominal area of a human body with improved reliability is provided. For slice images representing two or more cross-sections of an abdominal area of a subject, a ratio of a size of the subcutaneous fat region and the visceral fat region in the abdominal area to the abdominal area is calculated. Then, the ratio obtained for the subject is compared with a corresponding ratio obtained in advance for a human body model to acquire the diagnostic index indicating the condition of obesity of the subject.