  We discuss one-loop contributions of supersymmetry (SUSY) particles to the
process $e^+e^- \to W^+W^-$ in the minimal supersymmetric standard model. The
calculation is tested by using the sum rule among the form factors, which is
based on the BRS invariance. The overall normalization factor of the amplitudes
is tested by using the decoupling property of the SUSY particles in the large
mass limit. The correction due to the one-loop effect of the sfermions can be
of a few times $\pm 0.1$ % level, whereas that of the charginos and neutralinos
can be order 1%. We also study the CP-violating effects in the chargino and
neutralino sector.
