  One-loop contributions of super-partner particles to $W$-pair production at
$e^+e^-$ collision are discussed in the MSSM. To obtain trustworthy results we
test our calculation using three methods: (1) sum rules among form factors
which result from the BRS invariance, (2) the decoupling theorem, (3) the
high-energy stability. We examine the corrections taking into account
constraints from the direct search experiments and the precision data. The
results for the sfermion contributions are presented.
