We report on the feasibility of measuring the top Yukawa coupling in the process: $e^+e^-to tbar{t}H$. 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 $tbar{t}H$ event generator and carried out Monte Carlo simulations. Our results show that $tbar{t}H$ 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: $(-0.8,+0.3)$.
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