Recently, industrial robots are widely employed in various fields for automation. However, some assembly tasks still need intervention of human workers, because of the complicated assembly skills involved. In order to assist the assembly tasks, some industrial partner robot have been introduced into the assembly line. These devices are designed merely for supporting the weight of the heavy parts, and sometimes they embarrass the human work from applying their skills. In this research we propose a new system in which robots and human workers cooperate complementarily in an assembly task. This paper describes a prototype robot designed following this idea.