Nuclear waste contains a significant amount of long-lived non-gamma-emitting nuclei such as 129I and 14C. A method of nondestructive detection for monitoring long-lived waste products is proposed as an application of the (,n) reaction. This method is useful for surveying long-lived "difficult-to-measure" nuclides, e.g., 129I. Iodine-128 produced from the reaction of 129I(,n)128I emits gamma rays that can easily be measured by a gamma-ray counter. We measured the inclusive photonuclear 129I(,n)128I reaction cross section induced by bremsstrahlung photons. The photons were produced at a Ta target bombarded by 30-MeV electrons from a linear accelerator. The intensity of the slow neutrons was considered in the reactions of 127I(n, )128I and 129I(n, )130I. The activity of 128I was measured by a high-purity germanium spectrometer. The gamma-ray flux and the neutron flux were calculated using the EGS and MCNP codes, respectively. The average activation cross section of the 129I(,n)128I reaction had a 12% deviation from the evaluated International Atomic Energy Agency photonuclear data.