An image detector is disposed behind a grid. The image detector has normal pixels and measurement pixels. Out of a group of measurement pixels based on which an average value of dose measurement signals is calculated, a [C/D] number of measurement pixels are disposed or chosen in a cycle Z=(R×C)±D. Wherein, C represents a cycle of a repetition pattern appearing in an arrangement direction of X-ray transparent layers and X-ray absorbing layers in an X-ray image of the grid, and is represented in units of the number of pixels. R represents a natural number of 0 or more. D represents an integer less than the cycle C. [C/D] represents a maximum integer equal to or less than C/D. Provided that at least the [C/D] number of measurement pixels are shifted C occasions by one pixel, if D=1, the average value of the dose measurement signals is invariable without any variations.