Thin films of a new transparent oxide semiconductor 12CaO.7Al(2)O(3) consisting of subnanometer-sized cages (see Figure inset) have been fabricated, and their light-induced insulator-conductor conversion is described. Proton implantation at a fluence of 1 x 10(18) cm(-2) followed by UV-light irradiation increases the electrical conductivity by more than eleven orders of magnitude to the largest value similar to10 S cm(-1) (see Figure). The conducting state is erasable by heating.