The goal of this study is developing a virtual environment for collaborative work in which a human-shaped virtual agent would educate a trainee about the complicated tasks such as machine maintenance work and plant operation. To develop the virtual environment, some elemental technologies are required, and in this study a new method has been proposed for modeling the virtual agent's behavior by using Petri-net. Moreover, the design support system for constructing Petri-net through Graphical User Interface has been developed. As the result, it was confirmed that the design support system could make it efficient to model and decide the virtual agent's behavior.