  We study string scattering amplitudes by using the deformed cubic string
field theory which is equivalent to the string field theory in the proper-time
gauge. The four-string scattering amplitudes with three tachyons and an
arbitrary string state are calculated. The string field theory yields the
string scattering amplitudes evaluated on the world sheet of string scattering
whereas the coventional method, based on the first quantized theory brings us
the string scattering amplitudes defined on the upper half plane. For the
highest spin states, generated by the primary operators, both calculations are
in perfect agreement. In this case, the string scattering amplitudes are
invariant under the conformal transformation, which maps the string world sheet
onto the upper half plane. If the external string states are general massive
states, generated by non-primary field operators, we need to take into account
carefully the conformal transformation between the world sheet and the upper
half plane. We show by an explicit calculation that the string scattering
amplitudes calculated by using the deformed cubic string field theory transform
into those of the first quantized theory on the upper half plane by the
conformal transformation, generated by the Schwarz-Christoffel mapping.
