Provided is an electronic underlay with a wireless transmission function, designed not to sense a hand holding a writing tool at the time of writing on a sheet. The electronic underlay with a wireless transmission function can be laid under a sheet to perform writing with a writing tool. The electronic underlay is provided with: a sheet-like optical waveguide formed by sandwiching lattice-like cores between a sheet-like under cladding layer and a sheet-like over cladding layer; a light emitting element connected to one end surface of the core; a light receiving element connected to another end surface of the core; and wireless transmission means for wirelessly transmitting inputted letters as electronic data. An elasticity modulus of the core is larger than an elasticity modulus of the under cladding layer and the over cladding layer.