The objective of this study is to develop ``Virtual Collaborator¡Ç¡Ç as a new type of human-machine interface, which works as ``intelligent interface agent¡Ç¡Ç to help machine operators controlling large scale machine system such as power plant. A prototype system of the virtual collaborator has been developed, and the major functions of the present virtual collaborator confirmed by a dynamic simulation of human-machine interaction on a distributed computer network are that when an anomaly occurs in a virtual nuclear power plant, the virtual collaborator can automatically detect it, diagnose the root cause, and operate the control panel in accordance with emergency operation procedures, although it has no bi-directional communication function. The method of how to use voice communication are discussed in the paper with respect to the relation between human and the virtual collaborator as interface agent for plant system.