  We present measurements of CP-violating parameters in $B^0(\bar{B}{}^0) \to
\eta^\prime K_S^0$ and $B^{\pm} \to \eta' K^{\pm}$ decays based on a 41.8
fb$^{-1}$ data sample collected at the $\Upsilon(4S)$ resonance with the Belle
detector at the KEKB asymmetric-energy $e^{+} e^{-}$ collider. We fully
reconstruct one neutral $B$ meson as a $B^0(\bar{B}{}^0) \to \eta^\prime K_S^0$
CP eigenstate and identify the flavor of the accompanying $B$ from its decay
products. From the distribution of time intervals between pairs of $B$ meson
decay points, we obtain the CP-violating asymmetry parameters ${\mathcal
S}_{\eta^\prime K_S^0} = 0.28\pm0.55(stat)^{+0.07}_{-0.08}(syst)$, and
${\mathcal A}_{\eta^\prime K_S^0} = 0.13\pm0.32(stat)^{+0.09}_{-0.06}(syst)$.
We also reconstruct charged $B^{\pm} \to \eta^\prime K^{\pm}$ decays and
determine a direct-CP violating asymmetry value of ${\mathcal A}_{\eta^\prime
K^\pm}=(-1.5\pm7.0(stat)\pm0.9(syst))%$.
