A superconducting radiation detector relies upon the abruptness of a superconducting transition edge to converts a slight heat generated by an X-ray into a high signal current and uses an electrothermal self-feedback mechanism to provide a high energy resolution and a high counting rate. A calorimeter incorporating such a radiation detector has an absorber for absorbing X-rays, a resistor formed of a superconductor provided under the absorber and having a resistance value that varies with heat generated in the absorber, superconducting wires for connecting the resistor to an external current detector, a membrane on which the resistor is provided, and an insulating film provided between the resistor and the absorber and having at least one hole penetrating therethrough, the resistor and the absorber being in contact through the hole.