A radioactive ray detecting device comprises a superconductor element formed without an underlying substrate, an oxide film formed on a surface of the superconductor element, and a superconductor thin film formed on the oxide film. The oxide film and the superconductor thin film cover substantially an entire periphery of the surface of the superconductor element. The semiconductor element may be formed of a polycrystalline material such as aluminum, or a single crystal superconductor such as one containing tin, lead, niobium or tantalum, and may have a circular cross section or a rectangular cross section.