C4 level cord injury patients can't perform upper limb motion by themselves. Therefore, they need total assistance by care worker in daily life. It disturbs their independence and is burden for care workers. In addition, not moving upper limb makes joint contracture, muscle atrophy and QOL problems, further they can't work on a hobby with upper limb motion. In this study, we support C4 level cervical cord injury patient's upper limb motion. Electromyogram (EMG) signal is used to obtain their intention, then the proposed motion assist robot supports flexion and extension motion at elbow joint as the user intends. Using frequency analysis of the EMG signal, the patient's intention is estimated in real time, and the coefficient of viscosity in admittance controller is varied. Using proposed motion assist method, we realized the arbitrary upper limb motion of C4 level cervical cord injury person.