Abstract: For an open shop problem, if the principle of avoiding unnecessary idleness is applied to arrange available jobs for the schedule construction, a dense schedule is obtained. It is conjectured that the makespan of any dense schedule is at most 2-1/ m times the optimal makespan, where m is the number of machines. In this paper, we introduce the concepts of last job group and the related machines, and prove that the conjecture holds for m =5.