Quality degradation of a radiograph obtained from a stimulable phosphor sheet used repeatedly for radiography can be suppressed by an image data correction apparatus. Light emission characteristic storage means stores light emission change characteristic data and section storage means stores a position and a range of each of areas in the sheet obtained by dividing the sheet. Cumulative radiation dose acquisition means obtains an accumulated radiation dose for each of the areas, and light emission change rate acquisition means finds a light emission change rate representing a change in an amount of emitted light in accordance with the accumulated radiation dose for each of the areas, by using the light emission change characteristic data. Correction means corrects a value of image data obtained by reading each of the areas, for the change in the amount of the light with reference to the light emission change rate.