A bonding method using adhesive sheets respectively containing first and second thermoplastic resins. The volume content VA1 of the first thermoplastic resin in the adhesive sheet and the volume content VA2 of the second thermoplastic resin in the adhesive sheet are from 60 vol % to 100 vol %. Change rates Vx1 and Vx2 represented by formulae below are less than 80%. VB1 is the volume content of the first thermoplastic resin in a layer in direct contact with the first adhesive layer, and VB2 is the volume content of the second thermoplastic resin in a layer in direct contact with the second adhesive layer. The method includes applying a high-frequency wave to the adhesive sheets between adherends to bond them together,

Vx1={(VA1−VB1)/VA1}×100  (Numerical Formula 1)

Vx2={(VA2−VB2)/VA2}×100  (Numerical Formula 2).

A bonding method using adhesive sheets respectively containing first and second thermoplastic resins. The volume content VA1 of the first thermoplastic resin in the adhesive sheet and the volume content VA2 of the second thermoplastic resin in the adhesive sheet are from 60 vol % to 100 vol %. Change rates Vx1 and Vx2 represented by formulae below are less than 80%. VB1 is the volume content of the first thermoplastic resin in a layer in direct contact with the first adhesive layer, and VB2 is the volume content of the second thermoplastic resin in a layer in direct contact with the second adhesive layer. The method includes applying a high-frequency wave to the adhesive sheets between adherends to bond them together,
Vx1={(VA1VB1)/VA1}100(Numerical Formula 1)
Vx2={(VA2-VB2)/VA2}100(Numerical Formula 2).