A task scheduler scheduling running units to execute a plurality of tasks is provided. The task scheduler includes a time control portion having a common time to control a state of the plurality of tasks, and a task calculator calculating a slack disappearance time for each of the plurality of tasks. An arrival time of one of the plurality of tasks is defined as T. A deadline time representing when the one of the plurality of tasks is required to be completed is defined as D. A worst case execution time predicted to be required for a completion of the one of the plurality of tasks is defined as W. A current elapsed time is defined as C. The slack disappearance time is expressed by S=T+D−W+C. A task having an earliest slack disappearance time from among the plurality of tasks is scheduled to be preferentially executed.