A radioactivity analyzing apparatus for analyzing a radionuclide contained in a sample material, comprising: a radiation detector that detects a radiation to be detected emitted from a sample material; and a radiation analyzer configured to analyze the radiation based on an output from the radiation detector; the radiation analyzer including: a pulse-height analyzer configured to extract a pulse-height distribution from a pulse signal depending on the radiation, the pulse signal being output from the radiation detector, characterized by further comprising an inverse-problem operator configured to perform an inverse-problem solution of the pulse-height distribution to extract an energy spectrum of the radiation; and a deterioration detector configured to determine a deterioration state of the radiation detector based on the energy spectrum, wherein the deterioration detector is configured to determine the deterioration state based on a noise-generation ratio in the extracted energy spectrum, the deterioration detector is configured to determine that the deterioration has progressed when the noise-generation ratio is higher than a previous one, wherein the noise-generation ratio is defined as the ratio of an integrated count number of noise calculated by defining the central energy in the energy spectrum of interest and covering sub-ranges either side of this central energy, and making a count number thereof, to the count number at the energy of interest, which is the central energy in the energy spectrum of interest. a radiation detector that detects a radiation to be detected emitted from a sample material; and a radiation analyzer configured to analyze the radiation based on an output from the radiation detector; the radiation analyzer including: a pulse-height analyzer configured to extract a pulse-height distribution from a pulse signal depending on the radiation, the pulse signal being output from the radiation detector, characterized by further comprising an inverse-problem operator configured to perform an inverse-problem solution of the pulse-height distribution to extract an energy spectrum of the radiation; and a deterioration detector configured to determine a deterioration state of the radiation detector based on the energy spectrum, wherein the deterioration detector is configured to determine the deterioration state based on a noise-generation ratio in the extracted energy spectrum, the deterioration detector is configured to determine that the deterioration has progressed when the noise-generation ratio is higher than a previous one, wherein the noise-generation ratio is defined as the ratio of an integrated count number of noise calculated by defining the central energy in the energy spectrum of interest and covering sub-ranges either side of this central energy, and making a count number thereof, to the count number at the energy of interest, which is the central energy in the energy spectrum of interest. a pulse-height analyzer configured to extract a pulse-height distribution from a pulse signal depending on the radiation, the pulse signal being output from the radiation detector, characterized by further comprising an inverse-problem operator configured to perform an inverse-problem solution of the pulse-height distribution to extract an energy spectrum of the radiation; and a deterioration detector configured to determine a deterioration state of the radiation detector based on the energy spectrum, wherein the deterioration detector is configured to determine the deterioration state based on a noise-generation ratio in the extracted energy spectrum, the deterioration detector is configured to determine that the deterioration has progressed when the noise-generation ratio is higher than a previous one, wherein the noise-generation ratio is defined as the ratio of an integrated count number of noise calculated by defining the central energy in the energy spectrum of interest and covering sub-ranges either side of this central energy, and making a count number thereof, to the count number at the energy of interest, which is the central energy in the energy spectrum of interest.