  We made a narrowband NB973 (bandwidth of 200A at 9755A) imaging of the Subaru
Deep Field (SDF) and found two z=7 Lyman alpha emitter (LAE) candidates down to
NB973=24.9. Carrying out deep follow-up spectroscopy, we identified one of them
as a real z=6.96 LAE. This has shown that galaxy formation was in progress just
750 Myr after the Big Bang. Meanwhile, the Lyman alpha line luminosity function
of LAE is known to decline from z=5.7 to 6.6 in the SDF. L* at z=6.6 is 40-60%
of that at z=5.7. We also confirm that the number density of z=7 LAE is only
17% of the density at z=6.6 comparing the latest SDF LAE samples. This series
of significant decreases in LAE density with increasing redshift can be the
result of galaxy evolution during these epochs. However, using the UV continuum
luminosity functions of LAEs, those of Lyman break galaxies, and a LAE
evolution model based on the hierarchical clustering, we find that galaxy
evolution alone cannot explain all the decrease in density. This extra density
deficit can be interpreted as the attenuation of the Lyman alpha photons from
LAEs due to a rapid evolution of neutral hydrogen fraction during the ongoing
cosmic reionization at z~6.6-7.
