The invention provides a method which predicts a ratio of the increase in frictional resistance of a rough surface in a simple manner, quickly and without variations in predicted results among individuals. The method for predicting the frictional resistance of a rough surface having a variation in roughness wavelength, and being in contact with a fluid flowing at varied velocities includes evaluating the total projected area of all prominent peaks standing out above the viscous sublayer thickness per unit area, and calculating the friction increase ratio FIR (%) or the frictional resistance increase Δτ.
