Recently, industrial robots are widely employed in various fields. However, some assembly tasks are still manually completed because of complicate assembly sequences and requirements of high accuracy positioning. Therefore, we propose a new system in which human workers cooperate with assistant robot complementarily in assembling a heavy mechanical part. This paper describes the prototype of a gravity compensation mechanism and a gripper employed in this robot. The gravity compensation mechanism is designed to relieve the burden of worker for holding the mechanical part. The developed gripper is characterized by its implementation of non-force closure grasping. It allows to change the attitude of the payload in the envelope of the gripper closure.