PURPOSE: To enable to simplify a frame construction with a smaller package capacity of a data collection part by selecting signals from a plurality of channels of a detector sequentially to obtain a difference of a plurality of digital signals adjacent or close to each other.

CONSTITUTION: A frame package part A has a surface detector in which light- voltage transducers are arranged two-dimensionally to convert an X-ray dose from an X-ray bulb 1 into a voltage and an amplification part 2 to amplify outputs a1M-aNM of N×M pieces of X-ray detector. Signals b1M-bNM amplified are selected with M pieces of mutiplexer selected in sequence at each channel and a signal (c) thereof is converted into a digital signal (d) with an A/D converter 5. A differential compressor 6 determines a difference of a plurality of digital signals (d) from a channel adjacent or close thereto and digital signals (e) compressed with a parallel-serial converter 7 undergo a parallel-series conversion. A serial signal (p) obtained is transferred with a slip ring 8.