This paper describes a system called DiamondTheater that supports creative activities in theater using a tabletop tangible interface. This system is used to aid in the planning of certain aspects of theater production, such as actor positioning, and sound and lighting cues. Without having some considerable experience in production, it is difficult to create a mental picture of an actual production. Thus, a miniature theatrical stage is reproduced on the tabletop surface to facilitate the user's creation of such a picture and the sharing of ideas. Users collaboratively construct a stage by placing small dolls to represent actors and many kinds of miniature stage sets. In addition, the system allows users to reproduce other aspects of theater production such as sound and lighting. We performed a user study of this system and demonstrated that DiamondTheater appears to effectively assist the user's activity in theater production design.