  We examine the Friedel sum rule which states that the "excess charge" due to
a single impurity potential in a metal is equal to a sum of phase shifts for
scatterings of electrons by the impurity. For finite volume, the ``excess
charge" is given by the difference between total numbers of levels in the Fermi
sea with and without the impurity potential. However, a sequence of the "excess
charge" for finite volume is not necessarily bounded in the infinite volume
limit, as was pointed out by Kirsch. In order to circumvent this difficulty, we
define "excess charge" directly for the infinite volume. The Friedel sum rule
is proven to hold for the "excess charge" thus defined.
