A method of manufacturing a film waveguide includes supplying a precursor consisting of monomer or oligomer of an elastomer having a flexural modulus after curing smaller than or equal to 1,000 MPa to a substrate; pressing a stamper on the precursor of the elastomer, applying pressure to the precursor of the elastomer by the stamper and thinning a film thickness of the precursor of the elastomer; forming a lower clad layer by curing the precursor of the elastomer; forming a core on the lower clad layer; and forming an upper clad layer on the lower clad layer and the core. A film waveguide has at least one of a lower clad layer or an upper clad layer formed by an elastomer having a flexural modulus smaller than or equal to 1,000 MPa. A sum of film thicknesses of the clad layers is less than or equal to 300 mm.