  The Szego kernel of a strictly pseudoconvex domain admits a singularity on
the boundary diagonal, which consists of a pole and logarithmic type
singularity. In this paper, we prove that the integral over the boundary of the
coefficient of the logarithmic singularity gives a biholomorphic invariant of a
domain, or a CR invariant of the boundary. We also show that the same invariant
appears as the coefficient of the logarithmic term of the volume expansion of
the domain with respect to the Bergman volume element.
