PURPOSE: To improve processing ability per unit time by respectively converting light within respective light quantity ranges obtained by dividing the entire light quantity range of the light which expresses a radiation picture into plural stepwise into a signal within the input signal range of a logarithmic amplifier which is excellent in responsiveness and reading it.

CONSTITUTION: A gain setting means 22 is successively controlled every light quantity range so that the light within the respective light quantity ranges obtained by dividing the entire light quantity range of the light which expresses the radiation picture into plural stepwise may be respectively converted into the signal within the input signal range of the logarithmic amplifier 26 which is excellent in responsiveness and reading is performed. As a result, logarithmic conversion is performed with complete accuracy even if a reading speed is increased. Though the reading must be performed plural times in order to read over the entire light quantity range, the reading speed can be improved even if a time for reading plural times is added. Thus, the processing ability per unit time can be improved.