The radiographic image conversion panel for mammography includes a phosphor layer formed on a rectangular substrate having first and second pairs of two parallel sides, and sealed with a moisture-proof protective film. The phosphor layer is positioned on the substrate such that a distance from at least one side of the first pair to an adjacent end of the phosphor layer is shorter than a critical bonding length being a shortest bonding length long enough to provide a predetermined level of moisture-proof effect and distances from two sides of the second pair to adjacent ends of the phosphor layer are not shorter than the critical length. A seal bonding layer is formed in areas of the second distance, and on a side surface having the at least one side or on the side surface and a rear surface of the substrate. The phosphor layer may be formed in a recess formed in the substrate.