In a radiation image read-out system in which a stimulable phosphor sheet carrying a radiation image stored thereon is exposed to stimulating rays, and light emitted from the phosphor sheet is photoelectrically read out to reproduce a visible image, the gradient of a decay curve of the phosphor sheet is detected by exposing the phosphor sheet to stimulating rays and detecting the light emitted therefrom. Preliminary read-out for reading out image input information stored on the phosphor sheet is then conducted by using stimulating rays having stimulation energy lower than stimulation energy of stimulating rays used in final read-out. On the basis of the decay curve gradient and the image input information, read-out conditions in the final read-out and image processing conditions are set. The final read-out is conducted within one hour after the phosphor sheet is exposed to the stimulating rays for detecting the decay curve gradient.