There is described an image inputting apparatus for inputting a radiation image that is formed on a radiation sensitive panel by projecting radial rays penetrated through a subject. The apparatus includes a detector to detect the radiation image projected onto the radiation sensitive panel and a sensitivity compensating section to automatically find a sensitivity compensation value, which is employed for compensating for a sensitivity of the detector so that a signal value of the radiation image detected by the detector is substantially equal to a predetermined ideal value when a predetermined dose of the radial rays is irradiated onto the radiation sensitive panel. The apparatus also includes a storage section to store the predetermined ideal value of the signal value. The sensitivity compensating section finds an optimum sensitivity-offset value, which makes the signal value of the radiation image equal to the predetermined ideal value stored in the storage section.