A method of manufacturing an organic light-emitting display device is provided. An alignment master member is loaded on a moving unit. An organic layer deposition assembly is pre-aligned to the alignment master member. After the pre-aligning of the organic layer deposition assembly, a substrate is loaded on the moving unit. The organic layer deposition assembly is aligned to the substrate positioned as is after the loading of the substrate. An organic layer is formed on the substrate while the moving unit is moving along the moving direction. While the moving unit is moving along the moving direction, the organic layer deposition assembly is adjusted so that an interval between the organic layer deposition assembly and part of the substrate is maintained as substantially constant. The part of the substrate receives a deposition material emitted from the organic layer deposition assembly to form the organic layer.