An upper body motion measurement system 1 has a plurality of inertia sensor units 2, each of which incorporating an angular velocity sensor 4 and an acceleration sensor 5. The plurality of the inertia sensor units 2 is attached to places that are different from each other on the upper body of a subject P. Based on the detection outputs of the angular velocity sensor 4 and the acceleration sensor 5, the attitude of each of the inertia sensor units 2 is estimated, and the acceleration thereof is further estimated. The angular acceleration of the upper body of the subject P is estimated based on the estimated accelerations of the plurality of the inertia sensor units 2.
