  We investigate the critical endline of the finite temperature phase
transition of QCD around the SU(3)-flavor symmetric point at zero chemical
potential. We employ the renormalization-group improved Iwasaki gauge action
and non-perturbatively $O(a)$-improved Wilson-clover fermion action. The
critical endline is determined by using the intersection point of kurtosis,
employing the multi-parameter, multi-ensemble reweighting method to calculate
observables off the SU(3)-symmetric point, at the temporal size $N_{\rm T}$=6
and lattice spacing as low as $a \approx 0.19$ fm. We confirm that the slope of
the critical endline takes the value of $-2$, and find that the second
derivative is positive, at the SU(3)-flavor symmetric point on the Columbia
plot parametrized with the strange quark mass $m_s$ and degenerated up-down
quark mass $m_{\rm l}$.
