  We study one-loop effects of sfermions on helicity amplitudes for $\eeww$ in
the Minimal Supersymmetric Standard Model. The one-loop contributions are
calculated in the $\bar{\rm MS}$ renormalization scheme. In order to verify the
validity of the analytic calculation and the numerical program, the following
tests are performed. (i) The BRS sum rules hold exactly among the analytic
expressions of the form factors of the $\eeww$ amplitude and those of the
amplitudes where the external $W^\pm$ bosons are replaced by the corresponding
Goldstone bosons $\chi^\pm$, hence they hold within the expected accuracy of
the numerical program. (ii) The one-loop sfermion contribution to the
amplitudes decouple in the heavy mass limit. This property is used to test the
overall normalization of the amplitudes. In order to observe the analytically
exact decoupling, the amplitudes are expanded by the $\bar{\rm MS}$ couplings
of the Standard Model. (iii) The high-energy analytic formulas of the helicity
amplitudes, which are verified by using the equivalence theorem analytically,
are used for the numerical test of the high energy behavior of the amplitudes.
We then investigate the magnitude of the one-loop effects on each helicity
amplitude which may be measured by experiments at future linear colliders.
Under the constraint from available precision data, the one-loop corrections to
a few helicity amplitudes (for example, the longitudinal-$W$-pair production)
can be at most from -0.8% to $+0.6$% in magnitude in the observables. The
corrections in the helicity-summed cross sections are smaller, typically a few
times $\pm 0.1$% at large scattering angles.
