  We apply the Gamow shell model to study $^{25-31}$F isotopes. As both
inter-nucleon correlations and continuum coupling are properly treated therein,
the structure shape of $^{31}$F at large distance can be analyzed precisely.
For this, one-nucleon densities, root-mean square radii and correlation
densities are calculated in neutron-rich fluorine isotopes. It is then
suggested that $^{31}$F exhibits a two-neutron halo structure, built from both
continuum coupling and nucleon-nucleon correlations.
