Surface roughness of strain-relaxed Si1-xGex layers grown on Si(100) surfaces by a two-step growth method has been investigated. It has been found that the roughness is much smaller than that of layers grown by a conventional one-step growth method. The roughness depends on the Ge fraction and is increased at higher Ge fractions. In solid-phase epitaxial (SPE) of the first layer at a high Ge fraction of x = 0.8, an increase in thickness brings about a marked decrease in the roughness of the first layer. On the other hand, at x = 0.3, the roughness increases as increasing the thickness, which is considered to be due to atomic rearrangement in SPE.