An acceleration vector is detected in a kept-still state of a measurement target portion such as thighs of a target person to which an inertial sensor is attached, and further, another acceleration vector is detected in a state in which the target person is allowed to carry out exercise such that a posture of the measurement target portion is caused to change in a direction around a Yb axis with a Yb-axis direction of the measurement target portion maintained in a direction that perpendicularly intersects a vertical direction, direction in a sensor coordinate system the Yb-axis direction of the measurement target portion corresponds to is identified on the basis of a cross product vector of the acceleration vector and another acceleration vector.