The invention relates to a radiation detector. The detector includes an insulating substrate, a thin-film layer made of semiconductor or insulator formed on the surface of the substrate, at least a pair of electrodes provided on the thin-film layer, voltage applying means for applying a voltage across the electrodes and current detection means for detecting current taken from the electrodes, wherein radiation is detected using the fact that conductance of the thin-film layer changes linearly with respect to radiation intensity due to irradiation with radiation. Preferably, the thin-film layer comprises a metallic oxide. The metallic oxide comprises either one or any combination of two or more selected from titanium oxide, aluminum oxide (alumina), zirconium oxide, iron oxide, zinc oxide, yttrium oxide, manganese oxide, neodymium oxide, ceric oxide, tin oxide, or strontium titanate.