  We study the issue of defining the discrete light-cone quantization (DLCQ) in
perturbative string theory as a light-like limit. While this limit is
unproblematic at the classical level, it is non-trivial at the quantum level
due to the divergences by the zero-mode loops. We reconsider this problem in
bosonic string theory. We construct the multi-loop scattering amplitudes in
both open and closed string theories by using the method of Kikkawa, Sakita and
Virasoro (KSV), and then we show that these scattering amplitudes are perfectly
well-defined in this limit. We also discuss the vacuum amplitudes of the string
theory. They are, however, ill-defined in the light-like limit due to the
zero-mode loop divergences, and hence we want supersymmetry to cure those
pathological divergences even in string theory.
