Provided is an information management system, esigned not to sense a hand holding a writing tool at the time of writing on a sheet. The information management system is provided with a personal computer which has a display, and an electronic underlay which is laid under a sheet to perform writing. 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 cores; and a light receiving element connected to another end surface of the core. An elasticity modulus of the core is larger than an elasticity modulus of the under cladding layer and of the over cladding layer.