PURPOSE: To make a relative sensitivity constant, by comparing outputs of plural photoelectron multipliers for the light of the same light source with one another to detect relative sensitivities of them.

CONSTITUTION: A phosphor sheet 1 is irradiated with a laser light 102 to read data in advance by a reading part 10 for advance read, and the light of accelerated phosphorescene emission of the sheet 1 is made incident to a photoconductive sheet 104 for advance read and is received by a photoelectron multiplier 105. Radiation picture information outputted from a multiplier 5 is inputted to a sensitivity detector 40, and the radiation energy maximum level of the sheet 1 is measured in parallel with advance read by a photoelectron multiplier 301 and is inputted to the detector 40. The output of a photoelectron multiplier 205 obtained from the light of emission in a reading part 20 for regular read similarly to advance read is inputted to the detector 40. Output values of multipliers 105, 205, and 301 are compared with one another by the detector to detect respective relative sensitivities. Thus, relative sensitivities of plural multipliers 105, 205 and 301 are detected easily and accurately.