  Density matrix renormalization group (DMRG) is applied to a (1+1)-dimensional
$\lambda\phi^4$ model to study spontaneous breakdown of discrete $Z_2$ symmetry
numerically. We obtain the critical coupling $(\lambda/\mu^2)_{\rm c}=59.89\pm
0.01$ and the critical exponent $\beta=0.1264\pm 0.0073$, which are consistent
with the Monte Carlo and the exact results, respectively. The results are based
on extrapolation to the continuum limit with lattice sizes $L=250,500$, and
1000. We show that the lattice size L=500 is sufficiently close to the the
limit $L\to\infty$ \cite{Sugihara:2004qr}.
