Anisotropic electrochemical etching of porous GaN structures by utilizing charge carriers generated by sub-bandgap absorption has been developed. Sub-bandgap light with a photon energy below the bandgap energy was transmitted through bulk GaN, but a certain type of photo-absorption additively occurred at the pore tip due to the Franz-Keldysh effect. The photocurrents observed when the reverse bias was applied to n-GaN were well reproduced by calculation taking into account the Frantz-Keldysh effect. The pore diameter was not changed that much, but the pore depth could be successfully controlled by the intensity and irradiation time of the sub-bandgap light. (C) The Author(s) 2019. Published by ECS.