A method of smoothing a solid surface with a gas cluster ion beam includes irradiating the solid surface with the gas cluster ion beam. The irradiating includes, when scratches which can be likened to a line-and-space pattern structure with widths and heights on the order of a submicrometer to micrometer are present on the solid surface, a process of emitting the gas cluster ion beam so as to expose substances, which remain on side-walls of the scratches due to lateral transferal caused by collisions with gas clusters, to other gas clusters, and the gas cluster ion beam diverges non-concentrically and/or non-uniformly.