To shorten the time to reach an optimum solution, to realize a optimized approximate solution, and to largely reduce the program correction part, by merging a search algorithm having the total optimization as its feature with a heuristic logic having the partial optimization as its feature.

A tentative solution calculation part 1 calculates an initial process plan as a tentative solution by a simple logic defined based on the quantity needed for production and accordant with a production plan. A next candidate solution calculation part 2 calculates the next candidate solution by adding the perturbation to the tentative solution. A solution evaluation value calculation part 3 calculates the evaluation value of the next candidate solution. A solution searching part 4 searches for the solution obtained by a heuristic logic based on the next candidate solution to calculate the evaluation value to the searched solution and also to decide whether or not this evaluation value should be used as the standard for the following processes. The reference solution is repetitively given to the part 2 in several times, and the solution having the highest evaluation value is decided as the optimum solution of the process plan.