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