  An indication of spontaneous symmetry breaking is found in the
two-dimensional $\lambda\phi^4$ model, where attention is paid to the
functional form of an effective action. An effective energy, which is an
effective action for a static field, is obtained as a functional of the
classical field from the ground state of the hamiltonian $H[J]$ interacting
with a constant external field. The energy and wavefunction of the ground state
are calculated in terms of DLCQ (Discretized Light-Cone Quantization) under
antiperiodic boundary conditions. A field configuration that is physically
meaningful is found as a solution of the quantum mechanical Euler-Lagrange
equation in the $J\to 0$ limit. It is shown that there exists a nonzero field
configuration in the broken phase of $Z_2$ symmetry because of a boundary
effect.
