A method for producing a sealed optical semiconductor device makes it possible to seal an optical semiconductor element using a sealing film. The method includes: placing a sealing film on an optical semiconductor element substrate on which an optical semiconductor element is placed within a pressure reduction chamber, and the pressure within the chamber is reduced; heating the film where at least the periphery of the film is thermally fused to the surface of the optical semiconductor element placement substrate; and a step in which the reduction of pressure within the chamber is released and the optical semiconductor element placement substrate is sealed by the film. The temperature T2 of the optical semiconductor element placement substrate when the reduction of pressure within the chamber is released is a temperature at which the film exhibits a tensile strength of 0.02-0.15 MPa and an elongation at break of 150-450%.