Abstract: A power control apparatus of a boiling water atomic reactor for transferring the atomic reactor to a low power stand-by operation when a load of a generator is rejected. The necessity of insertion of control rods into a core is decided in accordance with the state of the atomic reactor when a load rejection occurs. When the insertion of control rods is required selected control rods are inserted into the core and a core flow rate is reduced to transfer the atomic reactor to the low power stand-by operation. When the insertion of control rods is not necessary only the reduction of core flow rate is performed.