A schedule decision method acquires dependencies of execution sequences required for a plurality of sub tasks into which a first task has been divided; generates a plurality of sub task structure candidates that satisfy said dependencies and for which a plurality of processing devices execute said plurality of sub tasks; generates a plurality of schedule candidates by further assigning at least one second task to each of said sub task structure candidates; computes an effective degree that represents effectiveness of executions of said first task and said second task for each of said plurality of schedule candidates; and decides a schedule candidate used for the executions of said first task and said second task from said plurality of schedule candidates based on said effective degrees.