Reducing the frictional drag of aircraft is an important issue for realizing a supersonic passenger aircraft because its poor fuel efficiency is one of the major concerns. Due to large swept angle of main wing and high Reynolds and Mach numbers, boundary layer around the wing is three-dimensional and growth rate of crossflow instability tends to be very large at position close to the leading edge. This leads to laminar-to-turbulent transition upstream and significantly increases skin friction over the wing surface. For a transonic three-dimensional boundary layer, a direct numerical analysis shows that this crossflow-induced transition can be suppressed strongly by placing sinusoidal roughness elements (SREs) near the leading edge, if the shape parameters of SREs such as height and angle are chosen optimally. The induced distortion of mean flow profile contributes to stabilization of three-dimensional boundary layer, and thereby the transition position is shifted downstream. A wind tunnel experiment was also conducted and successfully verified the expected laminarizing effect of SREs on a swept flat plate at 30m/s wind speed. In this study, the same laminarizing method by SREs is numerically examined for supersonic three-dimensional boundary layers on flat plate. Because of the large growth rate of crossflow instability, it turns out to be difficult to suppress it by SREs for the case of large swept angle. Nevertheless, by improving the shape of SREs, the suppression effect and transition delay can be observed at swept angles 27 degrees and 46 degrees when the local Mach number near the leading edge is about 1.5. This result indicates the possibility of applying SREs on the supersonic aircraft's wing for drag reduction.