  We present an optical multicolor-imaging study of the galaxy cluster Abell
521 at $z = 0.25$, using Suprime-Cam on the Subaru Telescope, covering an area
of $32 \times 20$ arcmin$^2$ ($9.4 \times 5.8 h_{50}^{-2}$ Mpc$^2$ at $z =
0.25$). Our imaging data taken with both a narrow-band filter, $NB816$
($\lambda_0 = 8150$\AA and $\Delta \lambda = 120$\AA), and broad-band filters,
$B,V,R_{\rm C}, i^\prime$, and $z^\prime$ allow us to find 165 H$\alpha$
emitters. We obtain the H$\alpha$ luminosity function (LF) for the cluster
galaxies within 2 Mpc; the Schechter parameters are $\alpha = -0.75 \pm 0.23$,
$\phi^\star = 10^{-0.25 \pm 0.20}$ Mpc$^{-3}$, and $L^\star = 10^{42.03 \pm
0.17}$ erg s$^{-1}$. Although the faint end slope, $\alpha$, is consistent with
that of the local cluster H$\alpha$ LFs, the characteristic luminosity,
$L^\star$, is about 6 times (or $\approx 2$ mag) brighter. This strong
evolution implies that Abell 521 contains more active star-forming galaxies
than the local clusters, being consistent with the observed Butcher-Oemler
effect. However, the bright $L^\star$ of Abell 521 may be, at least in part,
due to the dynamical condition of this cluster.
