A forward/reverse mechanics calculation of an accurate model of a human body having bone geometrical data and muscle/cord/band data is carried out at high speed. When a new skeleton geometrical model is given, a mapping between the new skeleton geometrical model and a pre-defined normal body model representing a normal body is defined to automatically produce a new body model. A processing unit reads model data to be subjected to mechanics calculation, reads a produced force f of a wire/virtual link exerted on the body model, reads the angle, position and velocity of the current rigid body link, calculates the Jacobian J.sub.L of the length of each wire concerning the joint angle, converts the read produced force f of the muscle/cord/band into a generalized force .tau..sub.G according to the defined Jacobian J.sub.L, stores the generalized force, determines the acceleration of the whole body of a motion produced when the generalized force .tau..sub.G is exerted on the body and calculates the velocity and position of each rigid body link, and stores them.