  The ground state and low lying excitation of the spin 1/2 Heisenberg ladder
with antiferromagnetic leg ($J$) and ferromagnetic rung ($-\lambda J, \lambda
>0$) interaction is studied by means of the density matrix renormalization
group method. It is found that the state remains in the Haldane phase even for
small $\lambda \sim 0.02$ suggesting the continuous transition to the gapless
phase at $\lambda = 0$. The critical behavior for small $\lambda$ is studied by
the finite size scaling analysis. The result is consistent with the recent
field theoretical prediction.
