A priority control apparatus according to the present invention includes: an OS execution unit which executes first tasks that run on a first OS and second tasks that run on a second OS; a task priority obtainment unit which obtains the priority of an execution task which is a first task being executed by the OS execution unit and the priority of a requested task which is a second task whose execution is being requested to the OS execution unit; and a priority changing unit which, in the case where the priority of the requested task is higher than the priority of the execution task, changes the priorities of the first tasks to be lower than the priority of the requested task and higher than the next lower priority to the requested task among the second tasks, while maintaining the relative order of the priorities among the first tasks.