  We calculate the spectral function of the massive quark at finite temperature
(T) using a Yukawa model and show that the peak in the negative energy region
among the three-peaks found in a previous work for the massless quark is
largely suppressed. To explore the underlying mechanism of this behavior, we
also investigate the pole structure of the retarded Green function of the
quark. We will show the result only for the massless quark. We find the
residues of the poles corresponding the three-peaks are all comparable at
$T\sim m_b$. We also show that the multi-peak structure of the quark spectra is
well described in the pole approximation which indicates that the quasi-paricle
picture is valid in this $T$ region.
