Many people of all ages have sustained cervical cord injury in traffic accidents or sport accidents, and consequently suffered physical impairment. Among these individuals dysfunction of the upper limbs is a concern, although recovery from dysfunction is possible through rehabilitation. In this study, we developed an assistive robot for upper limb movement which has high effectiveness in rehabilitation. To achieve this, we devised an algorithm of a dynamic filter that decreases the noise and delay of the device for controlling the motion assist robot stably. This filter changes the cutoff frequency depending on the amount of input change. The robot assists movement only during elbow extension. The effectiveness of the proposed method is demonstrated through experiments.