1. An operation plan decision method in which an operation plan is determined to satisfy constraint conditions, the method comprising: executed by a computer, deriving an feasible solution by using a constraint violation minimization model; updating candidates for an optimum solution and adding the updated candidates to a candidate list by taking the derived feasible solution, as an initial value of a candidate for the optimum solution, and by using a time cross-section division model that is obtained by dividing an optimization model for each time cross-section; and selecting the optimum solution from the candidate list to which the updated candidates are added.