A cassette for a stimulable phosphor sheet, includes a casing for storing the stimulable phosphor sheet therein, a cover openably and closably mounted on a portion of the casing, and a light-shielding plate mounted on the remaining portion of the casing for cooperating with the cover in keeping the stored stimulable phosphor sheet in a light-shielded condition. The stimulable phosphor sheet can be taken out of the casing by opening the cover. The cover has at least one locking finger normally urged by a spring to engage the casing to lock the cover in a closed position. The locking finger may be displaced by a presser pin inserted through a hole defined in a side of the casing or a hole defined in a bottom of the casing for unlocking the cover from the casing.