The formation of two-neutron halo is described using the neutron-neutron (n-n) interaction fixed at the low-energy n-n scattering limit. This method is tested for loosely-bound two neutrons in 24O, where a good agreement with experimental data is found. It is applied to halo neutrons in 22C in two ways: with the 20C core being closed or correlated due to excitations from the closed core. This nn interaction is shown to be strong enough to produce a two-neutron halo in both cases, locating 22C on the drip line, while 21C remains unbound. A unique relation between the two neutron separation energy and the radius of neutron halo is presented. New predictions for the two neutron separation energy and the radius of neutron halo are given for 22C. The estimated halo radius is found to be consistent with a recent experimental data. The appearance of Efimov states is also discussed. Spectra of excited states in 22C are predicted. The n-n interaction used here is large compared to conventional shell-model interactions. Roles of three-body forces are discussed.