  Measurement of the Yukawa interaction between the top quark and the Higgs
boson should be useful to clarify the mechanism of fermion mass generation.
  We discuss the impact of non-standard interactions characterized by
dimension-six operators on the effective top Yukawa coupling.
  The cross section of the process $e^-e^+ \to W^-W^+ \nu \bar \nu \to t \bar t
\nu \bar \nu$ is calculated including these operators, and possible deviation
from the standard model prediction is evaluated under the constraint from
perturbative unitarity and current experimental data.
  We find that if the new physics scale is in a TeV region, the cross section
can be significantly enhanced due to the non-standard interactions.
  Such a large effect should be detectable at the International Linear
Collider.
