To accurately measure the muscle mass, body-fat ratio and/or other information of the subject, the human body is divided into nine segments including the trunk, right and left forearms, right and left upper arms, right and left thigh, and right and left crura. Four distal voltage-measuring points Pv are determined at the wrists and ankles, and four proximal voltage-measuring points Pv are determined at the elbows and knees. Four current-carrying electrodes and four measuring electrodes are used. First, the measuring electrodes are attached to distal points, and the impedances of four limbs and the trunk are measured. Then, the measuring electrodes are moved to proximal points, and the impedances of four limbs and the trunk are measured. From the measurement result, the measurement value is calculated for each segment. Then, using estimation formulae created by a regression analysis based on the data collected with an MRI, the body composition such as a muscle mass is estimated from the measurement values of the impedances and body specific information including the height, weight, etc.