We compute the full O ( α s 4 ) and O ( α s 4 log α s ) corrections to the potential V R ( r ) between the static color sources, where V R ( r ) is defined from the Wilson loop in a general representation R of a general gauge group G. We find a violation of the Casimir scaling of the potential, for the first time, at O ( α s 4 ) . The effect of the Casimir scaling violation is predicted to reduce the tangent of V R ( r ) / C R proportionally to specific color factors dependent on R. We study the sizes of the Casimir scaling violation for various R's in the case G = SU ( 3 ) . We find that they are well within the present bounds from lattice calculations, in the distance region where both perturbative and lattice computations of V R ( r ) are valid. We also discuss how to test the Casimir scaling violating effect.