Method for manufacturing sheet container

This sheet container manufacturing method is a method for manufacturing a sheet container (100) configured so as to be provided with one or more sheets including a main body constituting sheet (21), the sheet container (100) being provided with a storage region (17) in which contents (18) are stored, and a container main body (20) configured from the main body constituting sheet (21) and surrounding the storage region (17). The main body constituting sheet (21) has a main body sealing part (26) which is a joint part between the outer film layer (22) and the inner film layer (23), a non-joint part (24) in which the outer film layer (22) and the inner film layer (23) are partially non-joined, and a filling part (60) in which a filling material is sealed in the non-joint part (24). The method is provided with: a step for introducing a gas into the storage region (17) and expanding the container body (20); and a step for injecting a filler between the outer film layer (22) and the inner film layer (23) in the non