A motion of a model including a joint, at least one body part that rotates with respect to the joint, and a plurality of inertial sensors attached to each body part and measuring a rotational motion of the body part is tracked by obtaining a rotational matrix of a sensor coordinate system fixed to each of the inertial sensors with respect to an inertial coordinate system fixed to ground, by using a signal measured by the inertial sensor attached to each body part; obtaining a rotational matrix of the sensor coordinate systems with respect to a body part coordinate system fixed to each body part, by using an obtained rotational matrix value of the sensor coordinate system; obtaining a rotational matrix of each body part coordinate system with respect to the inertial coordinate system, by using a calculated rotational matrix of the sensor coordinate systems; and calculating a joint variable with respect to each body part, by using a calculated rotational matrix of the body part coordinate system.