A method of manufacturing a treated sheet includes preparing a resin layer composite including a resin sheet and a thermoplastic resin layer supported on one surface of the resin sheet, applying a dispersion liquid including a powdery thermoplastic resin raw material onto a surface of the thermoplastic resin layer opposite to the resin sheet, and fusing the thermoplastic resin raw material to the thermoplastic resin layer by irradiating, with light from a flash lamp, a coating of the thermoplastic resin raw material formed in the applying step. The thermoplastic resin raw material has a melting point higher than the glass transition point of a resin that is a primary material of the resin sheet.