A rigidity control device is provided to change the natural resonance of an entire building by varying the rigidity of selected structural members such as pillars, beams, braces or floor and pillar, wall and pillar or beam connection members, joints and the like. The rigidity of a structural member is controlled by restraining or releasing a member from restraint or by applying a controlled force to the member. These processes are selectively combined so that the rigidity of the whole building and each section thereof is controlled by a computer according to earthquake forecast analysis. Thus, the proper rigidity of a building is determined and provided responsive to earthquake detection to prevent the building from destructive resonance caused by earthquake tremors.