  The systematic change of shell structure in both weakly bound and resonant
neutron one-particle levels in nuclei towards the neutron drip line is
exhibited, solving the coupled equations derived from the Schr\"{o}dinger
equation in coordinate space with the correct asymptotic behaviour of wave
functions for $r \rightarrow \infty$. The change comes from the behaviour
unique in the one-particle motion with low orbital angular momenta compared
with that with high orbital angular momenta. The observed deformation of very
neutron-rich nuclei with $N \simgeq 20$ in the island of inversion is a natural
result of this changed shell structure, while a possible deformation of
neutron-drip-line nuclei with $N \approx 51$, which are not yet observed, is
suggested.
