  We critically examine the reliability of the dark matter clustering in
high-resolution cosmological $N$-body simulations on scales below the mean
separation length of particles. The particle discreteness effect imposes the
two fundamental limitations on those scales; the lack of the initial
fluctuation power and the finite mass resolution. We address this problem
applying the dark halo approach and are able to discuss separately how those
two limitations affect the dark matter clustering in $N$-body simulations at
early epochs. We find that limitations of the dark matter clustering are
primarily determined by the mass of particles. By a detailed comparison with
three major cosmological simulations, we also find that in order to reproduce a
proper amplitude of the dark matter clustering on small scales, halos with a
characteristic nonlinear mass, $M_{\rm NL}(z)$ defined by $\sigma_R(M_{\rm NL};
z)=1$, must be resolved in the simulation. This leads to a critical redshift
$z_{\rm crit}$ determined by $M_{\rm NL}(z_{\rm crit})=n_{\rm halo} m_{\rm
part}$ where $n_{\rm halo}$ is the number of particles necessary to resolve the
typical nonlinear mass halo ($\sim 10$). We conclude that, at least as far as
the two-point correlation functions are concerned, the dark matter clustering
in high-resolution $N$-body simulations on scales below the mean particle
separation is reliable down to the gravitational force resolution length only
for $z<z_{\rm crit}$, while it is strongly affected by the finite mass
resolution for $z>z_{\rm crit}$.
