1. A robot control apparatus for controlling a plurality of mobile robots each of which includes a position information acquisition device for acquiring an instantaneous position and is configured to carry out a predetermined task, the apparatus comprising: a robot information storage device in which the instantaneous position acquired by the position information acquisition device of each mobile robot are storable; a task information storage device in which task information is storable, the task information including an execution position and a priority for each task to be executed by a mobile robot responsible therefor; and an action optimization controller configured to generate an instruction for optimizing actions of the plurality of mobile robots so that any of the plurality of mobile robots carry out the tasks at a minimized cost, based upon locomotion plan information indicative of locomotion plans of the plurality of mobile robots, each of the locomotion plans including a route along which each mobile robot moves from an instantaneous position of the mobile robot stored in the robot information storage device to an execution position of a task stored in the task information storage device, wherein the action optimization controller includes: a locomotion plan explorer configured to sort the locomotion plans by costs for exploration; a locomotion plan selector configured to select at least one locomotion plan from the locomotion plans sorted in the locomotion plan explorer; a locomotion plan implementation cost calculator configured to calculate a locomotion plan implementation cost to be entailed when the plurality of mobile robots move in accordance with the locomotion plan information updated by the at least one locomotion plan selected in the locomotion plan selector; a distance calculator configured to calculate distances from the plurality of mobile robots to obstacles for each point of time during implementation of the locomotion plans or for each position on routes along which the plurality of mobile robots move, in accordance with the locomotion plan information updated by the at least one locomotion plan selected in the locomotion plan selector; an encounter possibility determiner configured to compare the distances calculated by the distance calculator with a predetermined value, and to determine that at least one of the plurality of mobile robots has a possibility of encounter with an obstacle if at least one of the distances is below the predetermined value; an encounter-derived cost calculator configured to calculate an encounter-derived cost if the encounter possibility determiner determines that at least one of the plurality of mobile robots has a possibility of encounter with an obstacle; an optimum route selector configured to determine optimum locomotion plan information such that the tasks are carried out at a minimized cost, based upon the locomotion plan implementation cost calculated by the locomotion plan implementation cost calculator and the encounter-derived cost calculated by the encounter-derived cost calculator; and a plan modification instruction generator configured to generate a plan modification instruction for causing at least one of the plurality of mobile robots to move in accordance with the optimum locomotion plan information determined by the optimum route selector.