  Dynamical chiral symmetry breaking in the DLCQ method is investigated in
detail using a chiral Yukawa model closely related to the Nambu-Jona-Lasinio
model. By classically solving three constraints characteristic of the
light-front formalism, we show that the chiral transformation defined on the
light front is equivalent to the usual one when bare mass is absent. A quantum
analysis demonstrates that a nonperturbative mean-field solution to the
``zero-mode constraint'' for a scalar boson (sigma) can develop a nonzero
condensate while a perturbative solution cannot. This description is due to our
identification of the ``zero-mode constraint'' with the gap equation. The
mean-field calculation clarifies unusual chiral transformation properties of
fermionic field, which resolves a seemingly inconsistency between triviality of
the null-plane chiral charge Q_5|0>=0 and nonzero condensate. We also calculate
masses of scalar and pseudoscalar bosons for both symmetric and broken phases,
and eventually derive the PCAC relation and nonconservation of Q_5 in the
broken phase.
