  We conducted a deep narrowband NB973 (FWHM = 200 A centered at 9755 A) survey
of z=7 Lyman alpha emitters (LAEs) in the Subaru/XMM-Newton Deep Survey Field,
using the fully depleted CCDs newly installed on the Subaru Telescope
Suprime-Cam, which is twice more sensitive to z=7 Lyman alpha at ~ 1 micron
than the previous CCDs. Reaching the depth 0.5 magnitude deeper than our
previous survey in the Subaru Deep Field that led to the discovery of a z=6.96
LAE, we detected three probable z=7 LAE candidates. Even if all the candidates
are real, the Lyman alpha luminosity function (LF) at z=7 shows a significant
deficit from the LF at z=5.7 determined by previous surveys. The LAE number and
Lyman alpha luminosity densities at z=7 is ~ 7.7-54% and ~5.5-39% of those at
z=5.7 to the Lyman alpha line luminosity limit of L(Ly-alpha) >~ 9.2 x 10^{42}
erg s^{-1}. This could be due to evolution of the LAE population at these
epochs as a recent galaxy evolution model predicts that the LAE modestly
evolves from z=5.7 to 7. However, even after correcting for this effect of
galaxy evolution on the decrease in LAE number density, the z=7 Lyman alpha LF
still shows a deficit from z=5.7 LF. This might reflect the attenuation of
Lyman alpha emission by neutral hydrogen remaining at the epoch of reionization
and suggests that reionization of the universe might not be complete yet at
z=7. If we attribute the density deficit to reionization, the intergalactic
medium (IGM) transmission for Lyman alpha photons at z=7 would be 0.4 <=
T_{Ly-alpha}^{IGM} <= 1, supporting the possible higher neutral fraction at the
earlier epochs at z > 6 suggested by the previous surveys of z=5.7-7 LAEs, z ~
6 quasars and z > 6 gamma-ray bursts.
