Corrected X-ray detection signals are obtained by removing lag-behind parts through a recursive computation based on initial values determined from lag signal value (step T2). Thus, the lag-behind parts can be removed by taking into consideration lag signal values remaining at a starting point of the recursive computation. The lag signal values are dependent on the characteristic of an FPD (flat panel X-ray detector). By removing the lag-behind parts, with the lag signal values taken into consideration, the lag-behind parts are removed from X-ray detection signals with increased accuracy, without being influenced by the characteristic of the FPD.
