  Density matrix renormalization group (DMRG) is applied to a (1+1)-dimensional
$\lambda\phi^4$ model. Spontaneous breakdown of discrete $Z_2$ symmetry is
studied numerically using vacuum wavefunctions. 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 $L\to\infty$.
