  We present the results of the extensive narrow-band survey of Ly\alpha\
emission-line objects at z=3.1 in the 1.38 deg^2 area surrounding the high
density region of star-forming galaxies at z=3.09 in the SSA22 field, as well
as in the 1.04 deg^2 area of the three separated general blank fields. In total
of 2161 Ly\alpha\ emitters, 1394 in the SSA22 fields and 767 in the general
fields, respectively, are detected to the narrow-band AB magnitude limit of
25.73, which corresponds to the line flux of 1.8 x 10^{-17} erg s^{-1} cm^{-2}
or luminosity of 1.5 x 10^{42} erg s^{-1} at z=3.1, above the observed
equivalent width threshold, 190\AA . The average surface number density of the
emitters at z=3.1 in the whole general fields above the thresholds is
0.20+-0.01 arcmin^{-2}. The SSA22 high-density region at z=3.09 whose peak
local density is 6 times the average is found to be the most prominent
outstanding structure in the whole surveyed area and is firmly identified as a
robust `protocluster' with the enough large sample. We also compared the
overdensity of the 100 arcmin^2 and 700 arcmin^2 areas which contain the
protocluster with the expected fluctuation of the dark matter as well as those
of the model galaxies in cosmological simulations. We found that the peak
height values of the overdensity correspond to be 8-10 times and 3-4 times of
the expected standard deviations of the counts of Ly\alpha emitters at z=3.1 in
the corresponding volume, respectively. We conclude that the structure at
z=3.09 in the SSA22 field is a very significant and rare density peak up to the
scale of 60 Mpc.
