1. A computer-based system for production schedule creation, comprising: a production simulator that simulates a production process expressing a production state and a production constraint of the production process; a mathematical expression model holding device that holds a mathematical expression model which uses a state equation created by acquiring information relating to creation of a production schedule to which attention is paid, and is a mathematical expresion model expressing the production state and the production constraint of the above described production process in a mathematical expression; and an optimization calculation device that performs optimization calculation processing by using a predetermined evaluation function for the above described mathematical expression model to calculate a production instruction for said production simulator, wherein the production schedule in the above described production process is created from a simulation result obtained by repeating processing of calculating a feedback gain by the above described optimization calculation processing, calculating the production instruction by using the feedback gain and the production state to supply the production instruction to said production simulator to proceed with simulation, obtaining a new production state, and calculating a new production instruction based on the new production state.