1. A radiation imaging system, comprising: a radiation imaging unit for performing a radiographing of radiation; a table storing unit for storing a table set with at least driving conditions of said radiation imaging unit to each operation mode of a plurality of operation modes selected by a selecting unit for selecting an operation mode for performing a radiographing from the plurality of operation modes; an image storing unit for storing an image for correction radiographed based on the driving conditions set with the table in a state in which no object exists to each operation mode of the plurality of operation modes; and an image processing unit configured to perform a gain correction processing of the radiographed object image, wherein said image processing unit performs the gain correction processing of the radiographed object image obtained based on the driving conditions set in the table of the operation mode selected by said selecting unit in a state in which the object exists using a corresponding image for correction extracted from said image storage unit based on the operation mode selected by said selecting unit.