1. A priority control apparatus which controls priorities of first tasks that run on a first operating system (OS) and second tasks that run on a second OS, said priority control apparatus comprising: an execution unit configured to execute the first tasks and the second tasks; a task priority obtainment unit configured to obtain a priority of an execution task and a priority of a requested task, the execution task being a first task currently being executed by said execution unit, and the requested task being a second task whose execution is being requested to said execution unit; a priority judgment unit configured to judge whether or not the priority of the requested task is higher than the priority of the execution task; and a priority changing unit configured to change the priorities of the first tasks to be lower than the priority of the requested task and higher than a priority of a task having a next lower priority to the requested task among the second tasks while maintaining a relative order of the priorities among the first tasks, when the priority of the requested task is higher than the priority of the execution task.