  We present a detailed analysis of the behavior of two--level correlation
function $R(s)$ in the disordered sample. We show that in the vicinity of the
Anderson transition as well as in $2d$, the variance of the number of levels in
an energy band of a width $E$ has a linear behavior as a function of $E$. This
is related with an ``anomalous'' contribution to the sum rule for $R(s)$, which
comes from the ``ballistic'' region of $s$.
