A preparing method of a radiation image conversion panel comprising: forming a stimulable phosphor layer on a support while the stimulable phosphor layer being unformed on a peripheral portion around the stimulable phosphor layer on the support; and sealing the stimulable phosphor layer by superimposing a protective layer on the support formed thereon the stimulable phosphor layer while providing an adhesive agent on the peripheral portion, wherein the sealing step is conducted under a condition controlling humidity so that a moisture content of the stimulable phosphor layer is lower than the moisture content maintaining equilibrium with an atmosphere having a water partial pressure of 2.66null103 Pa.