A fundamental frequency and at least one harmonic frequency of gridlines in a radiographic image are measured to form a filter for extracting the measured fundamental frequency component and harmonic frequency component. Filtering is performed on the radiographic image using the formed filter. Based on the obtained image data, a gridline model which takes the measured fundamental frequency and harmonic frequency into consideration is fitted to generate a grid image. The gridlines are removed from the radiographic image by the generated grid image.