Recent industrial globalization has increased importance of functional verification in product design considering various ways of usage. In particular, for shortening development time, it is desired to perform functional verification and necessary modification of the design plan in the conceptual design phase. From this point of view, a framework for such individual functional verification that can be applied to the conceptual design phase was proposed. In this framework, functions, behavior and physical phenomena caused by a way of usage are modeled by function decomposition tree, Petri net, and cause-and-effect graph, respectively. They are then integrated as an augmented Petri net, and analyses for verification are performed by qualitative simulation on the integrated model. Although fundamental feasibility of this framework was illustrated, there are issues to be discussed further for application to actual product design. A serious one is behavior modeling. In the Petri net behavior modeling, detailed ways have not been discussed and the modeling is fully dependent on the designer. The resultant model varies depending on the designers even if he/she has perceived the behavior itself the same, and verification result may be inaccurate/inadequate if the model is inappropriate. It is desirable to reduce the degree of this dependence as much as possible. In addition, it is difficult for unpracticed designers to represent behavior of the design plan as a Petri net, since, in general, designers are not familiar with Petri nets. It is necessary to make this behavior modeling easier for designers. This paper provides a detailed way of the Petri net behavior modeling so that any designers can generate a standard behavior model which works in the framework as successfully and easily as possible. Each behavior is described by a unitary Petri net and the behavior of the design plan is described as a modularly-structured Petri net composed of those units. The modular structure is beneficial to automatic generation of the Petri net behavior model from the behavior model which has been generated by the designer in advance as a flowchart, which most designers are familiar with. Detailed ways of generating a Petri net model from a flowchart model are also given. This is helpful for designers to generate the Petri net model. The proposed method was applied to an example and its effectiveness was proven.