A radiographing apparatus of an image radiographing system is equipped with plural sensors each detecting an amount of radiation transmitted through a breast in the course of radiographing, and it transmits sensor data showing an output value of each sensor to a control apparatus through a network. In the control apparatus, a mammary gland content rate corresponding to an output value of each sensor is acquired based on sensor data transmitted from the radiographing apparatus, then, a breast type is discriminated based on an average mammary gland content rate and on the state of distribution of mammary gland content rates, and image processing conditions are established based on the results of discrimination of the breast type. Then, based on the image processing conditions thus established, image processing is given to image data acquired from a reading apparatus.