Abstract: A radiation detecting device comprises a substrate, a first superconductor layer formed on the substrate, a barrier layer formed on the first superconductor layer, and a second superconductor layer formed on the barrier layer, a junction formed of the first superconductor layer, the barrier layer and the second superconductor layer receiving radiation for detection thereof, the first superconductor layer being a Ta layer, the barrier layer including a TaOx layer, an AlOx layer, a HfOx layer or a ZrOx layer, and the second superconductor layer being a Ta layer. A underlayer of Nb, V, W, Hf, Zr or Ti having a lattice constant approximate to that of Ta is formed directly below the first superconductor layer. A underlayer of V, W or Ti having a lattice constant approximate to that of Ta is formed directly below the second superconductor layer. An opening is formed in the substrate and the radiation stopping layer directly below the junction, radiation being incident to the junction through the opening.