A storage phosphor plate assembly includes at least one storage phosphor plate having a storage phosphor layer for storing X-ray information and a substrate layer on which the storage phosphor layer is provided. The substrate layer contains a permanent magnetic material, and a support plate contains a magnetic material, the at least one storage phosphor plate being held on the support plate due to magnetic forces. The invention further relates to an X ray cassette and a system for reading out storage phosphor plate. An easy and safe handling of storage phosphor plates of various sizes and/or forms, in particular during the read-out process in the read-out system, is achieved.