An image management system is proposed that can display the same image in the same appearance on each image display device connected to a network independent of individual hardware characteristics. When an image file server that manages diagnostic images transfers the diagnostic image to an image display device connected to the network, it transfers a gradation table describing a gradation correction curve corresponding to the type of display of the image display device to be a destination as well. On the side of the image display device, the diagnostic image will be displayed in the same appearance independent of local characteristics such as hardware or the like by performing luminance correction on the diagnostic image referring to the received gradation table.