Disclosed is a method for minimizing the fluctuations in thickness of a substrate during annealing and achieving the smoothing of the surface of the substrate. Specifically disclosed is a method for treating the surface of an SOI substrate, which comprises at least the steps of: treating the surface of the SOI substrate by a PACE method using a plasma or a GCIB method using a gas cluster ion beam; and annealing the treated SOI substrate by heating the treated substrate in an argon atmosphere or an inert gas atmosphere containing 4 vol% or less of hydrogen.
