1. A vehicle planning support system for supporting vehicle planning by displaying vehicle models on a screen, comprising: specification value input screen display means for displaying a specification value input screen for an input of specification values including dimensions and angles in said vehicle model; vehicle model data generating means for generating vehicle model data based on the specification values input on said specification value input screen and rule data correlating between determined specification of the vehicle model for maintaining compatibility of shapes and arrangements of predetermined parts of the vehicle model; 3D morphing screen display means for displaying said vehicle model as a deformable 3D-shaped 3D vehicle model based on the vehicle model data generated by the vehicle model generating means using a 3D morphing screen, and 2D morphing screen display means for displaying said vehicle model as a deformable 2D-shaped 2D vehicle model based on the vehicle model data generated by the vehicle model generating means using a 2D morphing screen, wherein when a predetermined specification value of the vehicle model is changed on said 2D morphing screen or said 3D morphing screen, said 3D morphing screen displays means or said 2D morphing screen displays means change a correlated-specification value based on the rule data and display the 3D vehicle model or the 2D vehicle model so as to deform shapes and/or to change arrangements of parts thereof in relation to said changed predetermined specification value and said changed correlated-specification value, wherein said vehicle model includes a plurality of models and said rule data includes a master-slave relationship data which determines master-slave relationships between said plurality of models, by said master-slave relationship data, when a specification value of an upper level model is changed in said 2D morphing screen or said 3D morphing screen, specification value associated with those changed specification values of a lower level model is also changed in tandem therewith, and when a specification value of a lower level model is changed in said 2D morphing screen or said 3D morphing screen, a specification value of an upper level model is not changed in tandem therewith.