Bonding method

Provided is a method for firmly bonding members of a power module even when the surface area of a bonding surface is large, the method comprising: an oxygen ion conductor layer formation step for forming an oxygen ion conductor layer on the surface of one of a first material to be bonded, which has a metal, and a second material to be bonded, which has a ceramic; an arrangement step in which the first material to be bonded and the second material to be bonded are arranged so as to be in contact with each other with the oxygen ion conductor layer interposed therebetween; a connection step for connecting the first material to be joined to one of the positive electrode side and the negative electrode side of the voltage application device, and connecting the second material to be joined to the other of the positive electrode side and the negative electrode side of the voltage application device; and a voltage application step for applying a voltage between the first material to be bonded and the second material