  We make a search for Halpha emitting galaxies at z=0.24 in the Subaru Deep
Field (SDF) using the archival data set obtained with the Subaru Telescope. We
carefully select Halpha emitters in the narrowband filter NB816, using B, V,
Rc, i', and z' broad-band colors. We obtain a sample of 258 emitting galaxies
with observed equivalent widths of (Halpha+[NII]6548,6584) greater than 12
angstrom. We also analyze a sample of Halpha emitters taken from the Sloan
Digital Sky Survey (SDSS) to constrain the luminous end of Halpha luminosity
function. Using the same selection criteria as for the SDF, and after excluding
AGNs, we obtain 317 Halpha emitting star-forming galaxies. Combining these two
samples of Halpha emitters found in both SDF and SDSS, we derive a Halpha
luminosity function with best-fit Schechter function parameters of alpha =
-1.31^+0.17_-0.17, log phi^* = -2.46^+0.34_-0.40 Mpc^-3, log L^* =
41.99^+0.08_-0.07 ergs s^-1. An extinction-corrected Halpha luminosity density
is 4.45^+2.96_-1.75 x 10^39 ergs s^-1 Mpc^-3. Using the Kennicutt relation
between the Halpha luminosity and star formation rate, the star formation rate
density in the survey volume is estimated as 0.035^+0.024_-0.014 M_sun yr^-1
Mpc^-3. The angular two-point correlation function of Halpha emitters over 875
arcmin^2 at z = 0.24 is well fitted by a power-law form with w(theta) =
0.047^+0.017_-0.013 theta^-0.66 +- 0.08, corresponding to the correlation
function of xi(r) = (r/2.6^+1.0_-0.8 Mpc)^(-1.66 +- 0.08). The small
correlation length of Halpha emitters may imply the weak clustering of active
star-forming galaxies.
