To calculate a radioactive rays exposure for irradiating a stimulable phosphor sheet from radiation image information compressing a signal level exceeding an image signal range of an object.

Stimulated luminescence stimulated in accordance with the radiation image information cumulatively recorded on the stimulable phosphor sheet is detected by a photomultiplier tube 30, and the radiation image information is read. A read signal So is logarithm converted by a logarithm converter 32, and whether or not a logarithm converted signal Sk is a level equivalent to a reference value (V1 and V2)+0.1 digit or above is decided by hysteresis comparators 40 and 42 with a hysteresis width of 0.2. At this time, when the signal Sk is the level equivalent to the reference value (V1 and V2)+0.1 digit or above, the level of the signal Sk is reduced respectively by equivalent to 0.15 digit much by reducing a high voltage applied to the photomultiplier tube 30. In such a manner, the radioactive rays exposure for irradiating the stimulable phosphor sheet is calculated based on the signal Sk that a dynamic range out of the image signal range of the radiation image information is compressed.