This research goal is to establish a new design theory of co-creative expression interface for promoting interpersonal relationship with others, from the conviction that Hand contact improvisation, which is to touch their hands and create embodied improvisational expression together, is useful for deepening relationship with heavily developmentally disabled children who are handicapped about symbolical and oral communication skills. In this paper, inspired by cocreation with movement and touching interaction in music basically, we developed sound-expression Hand contact improvisation interface to motivate children toward bodily expression. This interface is cylindrical device that has contact parts on its both ends that have an ability to measure internal load of each hand, and it present sound from inner device. Sounds are generated by the system using multi phases that represent melodic lines. Interactions of these phases are manipulated by hand load data that are emerged among bodily expressers. When we conducted experiments of bodily expression with this interface, we observed increase tendency of interactions about hand load. Additionally, once we bring the interface to the practical field, we found people including autistic children can interact and bodily express using this interface. We discuss with these results about usefulness of our design method for making inclusive interface that supporting awareness toward bodily expression and induce diverse co-creative expression.