A method of manufacturing an organic electroluminescence device whereby an encapsulating layer is formed by coating an encapsulating substrate with an encapsulating layer-forming composition and thermally processing the encapsulating substrate. A plasma treatment is performed on the encapsulating substrate having the encapsulating layer. A sealant is applied to at least one of the plasma treated encapsulating substrate and a substrate on which an organic electroluminescent unit including a first electrode, an organic layer, and a second electrode, which are sequentially stacked, is deposited. The sealing substrate and the substrate on which the organic electroluminescent unit is deposited are combined. Contaminants generated around the encapsulating layer and generated in the thermal process can be effectively removed by the cleaning process using plasma. Therefore, the interfacial adhesion between the sealant and the substrate is greatly improved, thereby preventing permeation of external air, moisture, etc., into the device.