Petri net model is a widely used versatile tool which can represent a discrete event system such as mechanical system, communication system and production system. A variety of optimization problems can be formulated as the optimal firing sequence problem for Petri Nets, because this model is excellent in generality and in expression ability. However, when the size of the targeted model becomes large, the calculation time to solve the optimization problem is remarkably increased. In this paper, cooperation of production and transportation in semiconductor manufacturing is pursued expressing them by Petri Net. And we propose an approximation method for solving the optimal firing sequence problem that achieves the efficiency improvement in the solution procedure decomposing the Petri Nets .