  We have performed a deep optical imaging survey using a narrowband filter
($NB921$) centered at $\lambda =$ 9196 \AA ~ together with $i^\prime$ and
$z^\prime$ broadband filters covering an 814 arcmin$^2$ area of the Subaru Deep
Field. We obtained a sample of 73 strong $NB921$-excess objects based on the
following two color criteria; $z^\prime - NB921 > 1$ and $i^\prime - z^\prime >
1.3$. We then obtained optical spectroscopy of nine objects in our
$NB921$-excess sample, and identified at least two Ly$\alpha$ emitters
at$z=6.541 \pm 0.002$ and $z=6.578 \pm 0.002$, each of which shows the
characteristic sharp cutoff together with the continuum depression at
wavelengths shortward of the line peak. The latter object is more distant than
HCM-6A at $z=6.56$ and thus this is the most distant known object found so far.
These new data allow us to estimate the first meaningful lower limit of the
star formation rate density beyond redshift 6; $\rho_{\rm SFR} \sim 5.2 \times
10^{-4} M_\odot$ yr$^{-1}$ Mpc$^{-3}$. Since it is expected that the actual
density is higher by a factor of several than this value, our new observation
reveals that a moderately high level of star formation activity already
occurred at $z \sim$ 6.6.
