Free-form shape modeling is a difficult problem in interactive computer graphics. In case of using parametric patches, intuitive control of free-form shapes is not easy for novice users. Modeling methods based on strict physical laws need considerable computation time to deform objects. These problems on free-form shape modeling can be solved by regarding the object as clay which shows well-known plastic deformation. If users can deal with objects in the same way as clay works, the shape modeling becomes very user-friendly. In this paper we propose the virtual clay model for interactive free-form shape modeling. We make use of a cellular automaton, and try to apply it to plastic deformation within a 3D voxel space. Each voxel is allocated a finite state automaton which repeats state transitions according to the Margolus neighbor's conditions. Virtual clay objects defined in such a functional voxel space are easily deformed by a press operation with a moving plate. The conservation of total mass is held through all deformation.