The goal of this study is to develop a ``virtual collaborato¡Ç¡Ç as a new type of human interface environment. The virtual collaborator is an intelligent agent realized in VR space, who can communicate naturally with human like humans do with each other. As the first step of this study, the authors have constructed a prototype virtual collaborator who can behave just like plant operator in the control room of nuclear power plant, although the present prototype has no communication functions with humans. At the present stage, the virtual collaborator can detect an anomaly, diagnose the root cause and operate the control panel in accordance to the operation manual in the virtual space, where the control room of the nuclear power plant is visualized. The prototype system is constructed as a distributed simulation system, which consists of four subsystems; (1) nuclear power plant simulator, (2) man-machine interface simulator, (3) human model simulator, and (4) human body motion simulator. These subsystems have been separately developed and have been combined afterwards.