To confirm the long-term reliability of Cu(In,Ga)Se-2 (CIGS) solar cells, we investigated the I-V and C-V characteristics during rests under irradiation or dark condition. Under irradiation, the test samples showed a little increase in efficiency (eta) and open-circuit voltage (T-oc) which showed their electrical durability to light irradiation. But the diode factor (n) and series resistance (R-s) showed large changes in value. Also, the built-in voltage (V-b) and density gradient (dN(A)/d(X)) in the CIGS layer calculated from the C-V characteristics showed distinct changes during the test. After Lt SUN irradiation, two samples in the same fabrication-lot showed new light absorption in the lower-energy range than sun the energy gap of CIGS. We explain the change of C-V characteristics for the samples under strong irradiation with a new model named "Junction retrograde" which can treat defect generation by irradiation to reduce the acceptor density in graded pn junction. This model for C-V analysis can be used to investigate the long-term reliability of CIGS solar cells under irradiation.