1. A computer based system for production schedule creation, comprising: a production simulator means for simulating a production process expressing a production state and a production constraint of the production process, being configured in a Petri net model or a graph model as a discrete system that moves a thing at each event; a mathematical expression model holding means for holding a mathematical expression model output by said production simulator means, wherein said mathematical expression model is configured to correspond to said production simulator means and is created by said production simulator means in each case that an event requiring a production instruction occurs using elements relating to creation of a production schedule from all or only part of the production state and the production constraint of the above described production process; and an optimization calculation means for performing optimization calculation processing by using said mathematical expression model and a predetermined evaluation function for the above described mathematical expression model, and calculating a production instruction for said production simulator means; wherein the production instruction obtained by said optimization calculation means is supplied to said production simulator means to cause it to execute simulation, wherein simulation result is output as a production schedule, wherein whenever a new event requiring a production instruction occurs, said production simulator means and said optimization calculation means are linked to each other so that creating said mathematical expression model by said production simulator means and outputting an instruction to said optimization calculation means to perform optimization calculation from said production simulator means is repeated to create the production schedule in the above described production process.