The shape of a model object specified by a motion processing is changed depending on the shape of a landform so that at least one (foot or hand) of part objects forming the model object does not sink under the surface of the landform. The model object is disposed at a height in which any part object to be located on a lower surface of the landform is in contact with the surface of the landform. When any other part object sinks under the surface of the landform due to such a disposition, the shape of the model object is changed by the inverse kinematics so that the other part object is brought into contact with the surface of the landform. When the model object is in its jumping state, a judgment point for determining a disposing height of the model object is fixed at the representative point in the model object. The jumping orbit is changed depending on the height of the landform by changing the disposing height of the model object depending on the height of the landform at the position of the representative point.