A detection unit (10) detects a joint angle and a joint torque for causing the joint of a subject (1) to undergo passive flexion and extension movement, and an arithmetic unit (50) performs arithmetic processing on an output signal from the detection unit. The arithmetic unit divides the relationship between the joint torque and the joint angle in at least one of the extension phase and the flexion phase into two or more portions including a proximal-side portion and a distal-side portion according to a joint angle, and obtains the elastic coefficient of the joint from the relationship between the joint torque and the joint angle in at least one of the distal-side portion and the proximal-side portion. This enables distinguishing between a healthy subject and a Parkinson's disease patient with a simple configuration, without the measurement of surface myoelectric potential.