[영문]Rasmusen et al. (1991) and Segal and Whinston (2000) show that an incumbent monopolist might prevent entry of a more efficient competitor by exploiting externalities among buyers. We show that their results hold only when downstream competition among buyers is weak. Under fierce downstream competition, if entry took place, a free buyer would become more competitive and increase its output and profits at the expense of buyers that sign an exclusive deal with the incumbent. Anticipating that orders from a single buyer would trigger entry, no buyer will sign the exclusive deal and entry will occur. This result is robust across different specifications of the game.