1. A production schedule creation device, 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 is created by acquiring information relating to creation of a production schedule to which attention is paid, with a time period previously set from a schedule creation starting time of the above described production process (schedule creation time period) as a target, and is a mathematical expression-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, and calculates 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 the production instruction for a time period previously set from present time by the above described optimization calculation processing (instruction calculation time period) to supply the production instruction to said production simulator, executing simulation for only a previously set time period (simulation time period) to determine a production schedule for a previously set time period (schedule determination time period), and setting a date and time immediately after the above described determined time period as a new schedule creation starting time and time to formulate a production schedule.