Method of manufacturing treated sheet and method of manufacturing resin multilayer substrate

A method of manufacturing a resin multilayer substrate, the method comprising:

• preparing a resin layer composite including a resin sheet, a thermoplastic resin layer supported on one surface of the resin sheet, and a conductive pattern disposed on a surface of the thermoplastic resin layer opposite to 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 so as to cover the conductive pattern;
• 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;
• removing the resin sheet from the resin layer composite after the fusing step;
• stacking another resin layer on a surface of the resin layer composite subjected to the step of removing the resin sheet, the surface being located at a side at which the thermoplastic resin raw material is fused;
• and
• integrating a stack including the resin layer composite and the other resin layer by applying heat and pressure to the stack;
• wherein
• the thermoplastic resin raw material has a melting point higher than a glass transition point of a resin that is a primary material of the resin sheet.