A radiation image forming unit has a stimulable phosphor sheet and a case. The stimulable phosphor sheet includes a phosphor layer and a frame covering the four sides of the phosphor layer. The frame has a recess, and a lead sheet for blocking backward scattered rays is replaceably mounted in the recess by a double-sided tape. The radiation image forming unit may be used without the lead sheet. The lead sheet may be replaced with another lead sheet having a different thickness.