  We present a measurement of the time-dependent $CP$ violation parameters in
$B^0\to\eta'K^0$ decays. The measurement is based on the full data sample
containing $772\times 10^6$ $B\bar{B}$ pairs collected at the $\Upsilon(4S)$
resonance using the Belle detector at the KEKB asymmetric-energy $e^+e^-$
collider. The measured values of the mixing-induced and direct $CP$ violation
parameters are: \begin{align} \sin 2 \phi^{\rm eff}_1 &= +0.68\pm 0.07 \pm
0.03, \nonumber \\ \mathcal{A}_{\eta'K^0} &= +0.03\pm 0.05\pm 0.04, \nonumber
\end{align} where the first uncertainty is statistical and the second is
systematic. The values obtained are the most accurate to date. Furthermore,
these results are consistent with our previous measurements and with the
world-average value of $\sin 2\phi_1$ measured in $B^0\to J/\psi K^0$ decays.}
