1. A grid foil shadow removing method for a radiographic apparatus having a synchronous grid with grid foil strips arranged at regular intervals so that grid foil shadows fall on middles of pixels which detect radiation, the grid foil shadow removing method comprising: an approximate fluoroscopic image calculating step for obtaining an approximate fluoroscopic image by extracting detection signal values of pixels free from influences of the grid foil shadows from a fluoroscopic image and carrying out an interpolation process thereon; a grid foil shadow image calculating step for obtaining a grid foil shadow image by determining a difference between the fluoroscopic image and the approximate fluoroscopic image; a foil shadow standard image calculating step for obtaining a foil shadow standard image by averaging the grid foil shadow image in a longitudinal direction of the grid foil shadows; and a foil shadow removing step for removing the grid foil shadows from the fluoroscopic image based on the foil shadow standard image.