PROBLEM TO BE SOLVED: To provide a method capable of appropriately estimating a relative posture relationship between a measurement target portion of a subject and an inertial sensor attached to the measurement target portion. SOLUTION: While maintaining a constant predetermined direction (Yb axis direction) of a measurement target part such as a thigh of a subject P to which an inertial sensor 10 is attached, the posture of the measurement target part is about the Yb axis. The subject is made to move so as to change in the direction, and the three-dimensional angular velocity vector seen by the sensor coordinate system CSs is detected at one or more sampling times during this movement. Based on the detected angular velocity vector, it is specified which direction the Yb axis direction of the measurement target portion is in the sensor coordinate system CSs. [Selection diagram] Fig. 4【課題】対象者の計測対象部位とこれに取付けられた慣性センサとの間の相対的な姿勢関係を適切に推定し得る方法を提供する。【解決手段】慣性センサ１０が取り付けられた対象者Ｐの大腿部等の計測対象部位の所定の方向（Ｙｂ軸方向）を一定に維持しつつ、該計測対象部位の姿勢をＹｂ軸周りの方向に変化させるように対象者の運動を行わせ、この運動中の１つ以上のサンプリング時刻で、センサ座標系ＣＳｓで見た３次元の角速度ベクトルを検出する。検出した角速度ベクトルに基づいて、計測対象部位のＹｂ軸方向が、センサ座標系ＣＳｓで見てどの方向のであるか特定する。【選択図】図４
