Creative activities, such as painting and music, are one source of satisfaction and fulfillment for people with disabilities. However, some individuals with a disability can not satisfactorily enjoy such activities because of involuntary movement or spasms. In this study, we developed a drawing assist system for patients with cerebral palsy of the tension athetosis type, who experience spasticity that makes it difficult for the assist system to distinguish involuntary movement from voluntary movement. We designed a variable filter to attenuate involuntary movement on the basis of the behavioral characteristics of the velocity component with respect to in voluntary movement. Our system enabled drawing based on the participant's own senses and motor control, even when experiencing involuntary movement.