  We investigate the boundary effect of the density matrix renormalization
group calculation (DMRG), which is an artifactual induction of
symmetry-breaking pseudo-long-range order and takes place when the long-range
quantum fluctuation cannot be properly included in the variational wave
function due to numerical limitation. The open boundary condition often used in
DMRG suffers from the boundary effect the most severely, which is directly
reflected in the distinct spatial modulations of the local physical quantity.
By contrast, the other boundary conditions such as the periodic one or the
sin^2-deformed interaction [A. Gendiar, R. Krcmar, and T. Nishino, Prog. Theor.
Phys. {\bf 122}, 953 (2009)] keep spatial homogeneity, and are relatively free
from the boundary effect. By comparing the numerical results of those various
boundary conditions, we show that the open boundary condition sometimes gives
unreliable results even after the finite-size scaling. We conclude that the
examination of the boundary condition dependence is required besides the usual
treatment based on the system size or accuracy dependence in cases where the
long-range quantum fluctuation is important.
