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 , NB 816 
( \lambda _ { 0 } = 8150 Å and \Delta \lambda = 120 Å ) , and broad-band filters , B,V,R _ { 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 .