1. A solid-state image pickup device for generating image data according to an incident X-ray image, comprising: a first substrate having a first pixel array with M×NA (M and NA are each an integer not less than 2) pixels each including a photodiode that are two-dimensionally arrayed in M rows and NA columns; a second substrate having a second pixel array with M×NB (NB is an integer not less than 2 and smaller than NA) pixels each including a photodiode that are two-dimensionally arrayed in M rows and NB columns, and a first column of which is arranged along an NA-th column of the first pixel array; (NA+NB) readout wiring lines disposed for each column of the first and second pixel arrays, and each connected to the photodiode included in the pixel of a corresponding column via a readout switch; a signal output section for holding a voltage value according to an amount of charge input through the readout wiring line, and outputting the held voltage value converted to a digital value by one or a plurality of analog/digital converters; and a scintillator for generating scintillation light in response to incident X-rays to convert the X-ray image into an optical image, and outputting the optical image to the first and second pixel arrays, wherein one or a plurality of consecutive columns including a first column of the first pixel array and one or a plurality of consecutive columns including an NB-th column of the second pixel array serve as a non-sensitive region shielded from incident X-rays, and the signal output section outputs the digital values corresponding to the respective columns from the first column to an n-th column (2≦n<NA) of the first pixel array, sequentially starting from the first column to the n-th column, or starting from the n-th column to the first column, and in parallel with this output, outputs the digital values corresponding to the respective columns from an (n+1)-th column of the first pixel array through the NA-th column and the first column of the second pixel array to the NB-th column, sequentially in a reverse order to that of the first column to the n-th column of the first pixel array.