PURPOSE: To lower an overlapping temperature while defoaming by irradiating a laminated semiconductor wafer with an electromagnetic wave of a wavelength range to be absorbed by water molecules at the time of annealing.

CONSTITUTION: Two semiconductor wafers 1a, 1b are first overlapped at the ambient temperature. Then, it is irradiated with an electromagnetic wave of a wavelength range to be absorbed by water molecules at the time of annealing. When only water molecules generated by dehydrating and condensing the wafers 1a, 1b are selectively heated, a diffusion coefficient of the molecules is raised, and the diffusion of the molecules is advanced. Thus, only the molecules can be heated by the irradiating wave without heating the wafer to a high temperature, the diffusion constant of the molecules is increased thereby to effectively diffuse the molecules. Accordingly generation of foams due to water on the overlap surfaces can be prevented.