It is therefore preferable to carry out high-temperature annealing at 1,100 to 1,300??? C. in a hydrogen gas or an inert gas such as argon gas, or in a mixed gas of these gases, prior to the ion implantation of boron, to thereby improve the surface roughness of the SOI layer 7??? to be ion-implanted (e.g., 0.3 nm or less on the RMS basis).