The error of a sensor in motion capture system is corrected. An acceleration sensor determines the direction of gravity (G1) in a still initial state. The direction G1 will never change always if a drift dose not occur. However, actually if an object to which an acceleration sensor is attached moves along a coordinate axis, the direction gradually changes due to a drift with time. According to the invention, after a given time has passed, the acceleration sensor is stopped to determine the direction of gravity. Referring to the data on this determination, the true direction of gravity to be judged is compared with the direction of gravity G1 influenced by the drift. The difference between them is assumed to be an error due to a drift, and the error is subtracted from the determined value to correct the error of the measurement value. Similarly to the direction of gravity, assuming that there is an error in any direction in the coordinate system, the errors are corrected, and more accurate motion determination is possible.