  We report on the feasibility of measuring the top Yukawa coupling in the
process: $e^+e^-\to t\bar{t}H$. This measurement is crucial to test the mass
generation mechanism for matter particles. Since the cross section for this
process attains its maximum around $\sqrt{s}=700 $GeV, most of the past studies
were done assuming this energy region. It has been pointed out, however, that
the QCD threshold correction enhances the cross section significantly and might
enable its measurement at $\sqrt{s}=500 $GeV, which will be accessible already
in the first phase of the ILC project. We have implemented this threshold
enhancement into our $t\bar{t}H$ event generator and carried out Monte Carlo
simulations. Our results show that $t\bar{t}H$ events can be observed with a
significance of $4.1\,\sigma$ with no beam polarization and $5.4 \sigma$ with
the $e^-$ and $e^+$ beam polarization combination: $(-0.8,+0.3)$.
