  We use 5000 cosmological N-body simulations of 1(Gpc/h)^3 box for the
concordance LCDM model in order to study the sampling variances of nonlinear
matter power spectrum. We show that the non-Gaussian errors can be important
even on large length scales relevant for baryon acoustic oscillations (BAO).
Our findings are (1) the non-Gaussian errors degrade the cumulative
signal-to-noise ratios (S/N) for the power spectrum amplitude by up to a factor
of 2 and 4 for redshifts z=1 and 0, respectively. (2) There is little
information on the power spectrum amplitudes in the quasi-nonlinear regime,
confirming the previous results. (3) The distribution of power spectrum
estimators at BAO scales, among the realizations, is well approximated by a
Gaussian distribution with variance that is given by the diagonal covariance
component. (4) For the redshift-space power spectrum, the degradation in S/N by
non-Gaussian errors is mitigated due to nonlinear redshift distortions. (5) For
an actual galaxy survey, the additional shot noise contamination compromises
the cosmological information inherent in the galaxy power spectrum, but also
mitigates the impact of non-Gaussian errors. The S/N is degraded by up to 30%
for a WFMOS-type survey. (6) The finite survey volume causes additional
non-Gaussian errors via the correlations of long-wavelength fluctuations with
the fluctuations we want to measure, further degrading the S/N values by about
30% even at high redshift z=3.
