An objective is to provide a radiation flat panel detector in which degradation of a phosphor layer property during aging is inhibited, the phosphor layer is protected from chemical alteration and physical impact, and sharpness is excellent. Also disclosed is a flat panel detector possessing a light-receiving element and provided thereon, a scintillator panel possessing a scintillator sheet having a phosphor layer provided on a substrate and provided thereon, a first protective film placed on the phosphor layer side and a second protective film placed on the substrate side to seal the scintillator sheet, wherein the light-receiving element is provided on the first protective film side of the scintillator panel; and the scintillator sheet is sealed at 5-8000 Pa.