A radiation imaging system includes a detector including a plurality of pixels which obtain pixel values corresponding to incident radiation transmitted through a subject, and an information processing unit configured to perform a process of estimating information on thicknesses of substances included in the subject using pixel values of an arbitrary one of the plurality of pixels, an average value of energy of radiation quanta of the arbitrary pixel calculated in accordance with the pixel values of the arbitrary pixel, a first table indicating the relationship between the pixel values of the arbitrary pixel and the thicknesses of the substances included in the subject, and a second table indicating the relationship between the average value of the energy of the radiation quanta of the arbitrary pixel and the thicknesses of the substances included in the subject.