To provide a diffusion bonding method by which sufficient bonding strength can be obtained and the deformation of the body to be bonded after diffusion bonding is reduced.

In the diffusion bonding method where the body 1 to be bonded is pressurized only to the lamination direction of the body 1 to be bonded by a pressurizing apparatus, a metal plate 3 (3a to 3n) is arranged in a state of being provided with a prescribed gap with the outer wall face 5 of the laminated body 1 to be bonded so as to surround the whole of the outer wall face 5 of the body 1 to be bonded, only the body 1 to be bonded is pressurized to the lamination direction of the body 1 to be bonded by the pressurizing apparatus, and the body 1 to be bonded is bonded while restricting the deformation of the body 1 to be bonded to a direction orthogonal to the pressurizing direction.