In capturing an image of a grid by an image detector, a measurement pixel that is not in the position of a specific point having a maximum or minimum value of an output signal is referred to as a first measurement pixel, and a measurement pixel that is in the position of the specific point is referred to as a second measurement pixel. The disposition of the first and second measurement pixels are determined so as to satisfy the following condition: fG/fN≠odd number, wherein fG is a grid frequency and fN is a Nyquist frequency of pixels; and in shifting the grid C times by one pixel, the number of the first measurement pixels is larger than that of the second measurement pixels at any time in the range of a cycle C of a repetition pattern appearing in the image.