  We show that the Nambu-Goldstone(NG) boson restricted on the light-front(LF)
can only exist if we regularize the theory by introducing the explicit breaking
NG-boson mass $m_{\pi}$. The NG-boson zero mode, when integrated over the LF,
must have a singular behavior $\sim 1/m^2_{\pi}$ in the symmetric limit of
$m^2_{\pi}\rightarrow 0$. In the discretized LF quantization this peculiarity
is clarified in terms of the zero-mode constraints in the linear $\sigma$
model. The LF charge annihilates the vacuum, while it is not conserved in the
symmetric limit in the NG phase.
