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. We have already proposed an approximation method that achieves the efficiency improvement in the solution procedure by decomposing the Timed Petri Net. In this paper, we apply this proposed method to lot scheduling problem in steel works and the utility of this method is verified.