Most elderly people staying in nursing homes have dementia, and various kinds of recreational programs are provided in order to prevent or ameliorate this problem. Robot therapy is a recreational program that has recently been introduced in some nursing homes. However, the effects of robot therapy are difficult to evaluate. In this study, we identified events occurring during therapy that are important in the proliferation and evolution of robot therapy. We also found that the presence of an intervener encourages elderly people to be more active.