Using the transport equation of radiation, the disturbance of radiation field due to the introduction of a detector into a medium can be precisely expressed as the influence of an equivalent radiation source in the region, where the detector exists, on the radiation field. This result makes it possible to precisely estimate the correct coefficient of the disturbad radiation field. On this basis, the disturbance of the radiation fields in the cases of an infinitely high rectangular cavity and a small spherical detector is evaluated.