  Using a simple model based on the knowledge of spherical and deformed
Woods-Saxon potentials, it is shown that the recent observation of halo
phenomena in $^{29}$F can be interpreted as an evidence for the prolate
deformation of the ground state of $^{29}$F. The prolate deformation is the
result of the shell structure, which is unique in one-neutron resonant levels,
in particular near degeneracy of the neutron 1$f_{7/2}$ and 2$p_{3/2}$ resonant
levels, together with the strong preference of prolate shape by the proton
number $Z$ = 9. On the other hand, in oxygen isotopes spherical shape is so
much favored by the proton number $Z$ = 8 that the presence of possible neutron
shell-structure may not make the system deformed. Thus, the strong preference
of particular shape by the proton numbers 8 and 9, respectively, together with
a considerable amount of the energy difference between the neutron $1d_{3/2}$
and $2s_{1/2}$ orbits in oxygen isotopes seems to play an important role in the
phenomena of oxygen neutron drip line anomaly, as was suggested by H. Sakurai
{\it et al.} in 1999.
