PURPOSE: To facilitate selection taking the various conditions which are optimum and peculiar to respective control objects into consideration when the respective objects are selected by a method wherein a combination among a plurality of the control objects whose total control value is the largest but not exceeds a control target value can be selected taking the priority orders of the respective control objects into consideration.

CONSTITUTION: A region where an identical effect can be expected when a control value is larger than a given target control value 21 or when the control value is lower than the given target control value 21 is defined as a dead zone region 22. The part of the region 22 lower than the target value is defined as a region I 24 and the part of the region 22 higher than the target value is defined as a region II 25. A lower control limit value 23 is predetermined as a region where the control effect of the target control value can not be expected and a region between the region I 24 and the lower control limit value 23 is defined as a region III 26 and a region lower than the lower control limit value 23 is defined as a region IV. Further, a region higher than the region II 25 is defined as a region V 28. Then priority orders 1-4 are given to respective control objects. If a plurality of combinations exist, the combinations whose total control values are largest are selected as the control objects and the combination which has the smaller number of low priority generators is selected as the control object. The control object selected by the above process is used as a control output.