  So far we have considered that a two-point string amplitude vanishes due to
the infinite volume of residual gauge symmetry. However recently
Erbin-Maldacena-Skliros have suggested that the two-point amplitude can have
non-zero value, because one can cancel the infinite volume by the infinity
coming from on-shell energy conservation. They derived the two-point function
by Fadeev-Popov method. In this paper we revisit this two-point string
amplitude in the operator formalism. We find the mostly BRST exact operator
which yields non-zero two-point amplitudes.
