  We present new results of a deep optical imaging survey using a narrowband
filter ($NB921$) centered at $\lambda =$ 9196 \AA ~ together with $B$, $V$,
$R$, $i^\prime$, and $z^\prime$ broadband filters in the sky area of the Subaru
Deep Field which has been promoted as one of legacy programs of the 8.2m Subaru
Telescope. We obtained a photometric sample of 58 Ly$\alpha$ emitter candidates
at $z \approx$ 6.5 -- 6.6 among $\sim 180$ strong $NB921$-excess ($z^\prime -
NB921 > 1.0$) objects together with a color criterion of $i^\prime - z^\prime >
1.3$. We then obtained optical spectra of 20 objects in our $NB921$-excess
sample and identified at least nine Ly$\alpha$ emitters at $z \sim 6.5$ -- 6.6
including the two emitters reported by Kodaira et al. (2003). Since our
Ly$\alpha$ emitter candidates are free from strong amplification of
gravitational lensing, we are able to discuss their observational properties
from a statistical point of view. Based on these new results, we obtain a lower
limit of the star formation rate density of $\rho_{\rm SFR} \simeq 5.5 \times
10^{-4}$ $h_{0.7}$ $M_\odot$ yr$^{-1}$ Mpc$^{-3}$ at $z \approx 6.6$, being
consistent with our previous estimate. We discuss the nature of star-formation
activity in galaxies beyond $z=6$.
