An extracting step (step S1) extracts uninfluenced pixels with a relatively high degree of certainty, while avoiding influences of random quantum noise as much as possible. An approximate fluoroscopic image is obtained based on such uninfluenced pixels (step S2). Thus, accuracy of the approximate fluoroscopic image can be improved over that of the prior art. Therefore, a grid foil shadow image (step S3) and a foil shadow standard image (step S4) calculated successively based on the approximate fluoroscopic image have improved accuracy over the prior art. As a result, while inhibiting influences of random quantum noise, a foil shadow removed image can be obtained which is free from artifacts due to distortion of a synchronous grid.