  To derive a new H$\alpha$ luminosity function and to understand the
clustering properties of star-forming galaxies at $z \approx 0.24$, we have
made a narrow-band imaging survey for H$\alpha$ emitting galaxies in the HST
COSMOS 2 square degree field. We used the narrow-band filter NB816 ($\lambda_c
= 8150$ \AA, $\Delta \lambda = 120$ \AA) and sampled H$\alpha$ emitters with
$EW_{\rm obs}(\rm H\alpha + [N\textsc{ii}]) > 12$ \AA in a redshift range
between $z=0.233$ and $z=0.251$ corresponding to a depth of 70 Mpc. We obtained
980 H$\alpha$ emitting galaxies in a sky area of 5540 arcmin$^2$, corresponding
to a survey volume of $3.1 \times 10^4 {\rm Mpc^3}$. We derive a H$\alpha$
luminosity function with a best-fit Schechter function parameter set of $\alpha
= -1.35^{+0.11}_{-0.13}$, $\log\phi_* = -2.65^{+0.27}_{-0.38}$, and $\log L_*
({\rm erg s^{-1}}) = 41.94^{+0.38}_{-0.23}$. The H$\alpha$ luminosity density
is $2.7^{+0.7}_{-0.6} \times 10^{39}$ ergs s$^{-1}$ Mpc$^{-3}$. After
subtracting the AGN contribution (15 %) to the H$\alpha$ luminosity density,
the star formation rate density is evaluated as $1.8^{+0.7}_{-0.4} \times
10^{-2}$ $M_{\sun}$ yr$^{-1}$ Mpc$^{-3}$. The angular two-point correlation
function of H$\alpha$ emitting galaxies of $\log L({\rm H\alpha}) > 39.8$ is
well fit by a power law form of $w(\theta) = 0.013^{+0.002}_{-0.001}
\theta^{-0.88 \pm 0.03}$, corresponding to the correlation function of $\xi(r)
= (r/1.9{\rm Mpc})^{-1.88}$. We also find that the H$\alpha$ emitters with
higher H$\alpha$ luminosity are more strongly clustered than those with lower
luminosity.
