A new type of slow‐relaxation phenomena is reported together with the conventional fatigue phenomenon in photoconductivity for undoped semi‐insulating liquid‐encapsulated Czochralski GaAs. First, conventional fatigue is induced by the ∼1.1‐eV light irradiation (primary light). Secondary‐light irradiation (∼0.8 eV) subsequently carried out is found to generate a rapid increase followed by a gradual exponential decrease in photoconductivity. The latter phenomenon shows very different characteristics from those of conventional fatigue, though the two phenomena are similar to each other. Through the detailed experimental studies, a configuration‐coordinate model is presented, involving a main deep levelXand its metastable excited stateX* with large lattice relaxation. This phenomenon is explained by the transition of electrons which occurs in part from levelX* excited by the preceding primary‐light irradiation, via levelX, to the conduction band as a result of secondary‐light irradiation.