A radiation flat panel detector is created 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. 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; the scintillator sheet is sealed at 5-8000 Pa; and the phosphor layer brought into contact with the first protective film and the light-receiving element plane brought into contact with the first protective film both have a surface roughness Ra of 0.05-0.8 .mu.m.