To decide the load distribution to thermal power generators in such a way that the total fuel cost of the generators becomes the lowest, by newly keeping the output band state of a solution and the outputs of the generators as the best solution when the evaluated value of the solution is improved, and storing necessary data at each means.

An output band state generating means 2 successively generates different output band states against a certain output band state by regarding the output band state as the present state. Then an optimum generator output value calculating means 3 finds the output of each generator that makes the sum total of the power generating costs of the generators the minimum, while meeting each restriction such as generator output upper and lower limit restrictions in a demand-and-supply balanced state and in output bands. In addition, a solution evaluation calculating means 4 calculates the sum total of the fuel costs found from the outputs of the generators and, when the outputs infringe the above- mentioned restrictions, the fuel costs obtained by converting the amounts of the infringement and evaluates the sum total. Successively, a best solution keeping means 5 newly keeps the output band state and generator outputs of the above-mentioned solution as the best solution when the evaluated value of the above-mentioned solution is farther improved than the kept best solution. By repeating the above-mentioned operations, the best solution is obtained.