1. A calculating device comprising: a memory apparatus that stores a search space specified with a state set and an action set, and a presumed minimax having an arbitrary value; and a processor configured to: sequentially select an action from the action set in the search space stored in the memory apparatus, and sequentially select each state in the state set corresponding to the selected action; acquire and store into the search space, an evaluation value of the combination of the selected state and the selected action; identify a maximum evaluation value among the evaluation values acquired for each of the selected actions; select an unselected state for the selected action when an evaluation value of the combination of the selected action and the selected state is at most the presumed minimax, or terminate selection of an unselected state for the selected action, and select an unselected action when an evaluation value of the selected action and the selected state is greater than the presumed minimax; determine as a minimax candidate, a lowest evaluation value among the maximum evaluation values identified for an action for which selection has not been terminated; judge whether termination of selection has occurred for all the actions when no action to be selected is present; determine as a minimax, the minimax candidate determined for an action for which selection has not been terminated when any action for which state selection has not been terminated is present, or else enlarge the presumed minimax and retry the calculation; and output the action for which the minimax has been determined.