In a method of manufacturing a radiation image storage phosphor plate, comprising in its phosphor layer a storage phosphor of the type having an alkali metal halide salt as a main component and a lanthanide as a dopant, the plate after having been manufactured is uniformly exposed, i.e., exposed with a constant intensity and with a homogeneous energy distribution over its whole surface area, in order to absorb an X-ray dose in the range from 1 to 10 Gy.