A radiation monitor, comprising: a radiation detection unit (11, 111) which includes a radiation detection element (11a, 111a), the radiation detection element (11a, 111a) sensing radiation and light, and emitting light of a predetermined light emission wavelength when receiving the radiation or the light; a light emission unit for emitting light of a wavelength different from the predetermined light emission wavelength; a wavelength selection unit which is configured to be set in a first mode, wherein in the first mode the wavelength selection unit passes the light of the predetermined light emission wavelength, and blocks the light from the light emission unit; an optical transmission line (15a - 15f, 151a) for transmitting the light from the light emission unit to the radiation detection unit (11, 111), and for transmitting the light from the radiation detection unit (11, 111) to the wavelength selection unit; a light detection unit for converting the light passing through the wavelength selection unit into an electric pulse; and a control unit for measuring a count rate of the electric pulse, the radiation monitor (100, 100A) is characterized in that the control unit is configured to determine whether at least the light emission unit is degraded on the basis of the count rate and the light intensity of the light emission unit, and the control unit is further configured to include a memory unit for storing first light intensity-count rate information indicating a relation between the light intensity of the light emission unit when it is already known that the radiation monitor (100, 100A) functions normally and the count rate in a state where the wavelength selection unit is set to the first mode, and the control unit is further configured to perform maintenance of the radiation monitor (100, 100A), wherein the wavelength selection unit is set to the first mode wherein during maintenance a dose rate of radiation incident on the radiation detection element (11a, 111a) is at a level of natural radiation and wherein the control unit during maintenance is configured to determine, on the basis of a setting value of a predetermined light intensity, that the light emission unit is degraded when an actual light intensity of the light emission unit is smaller than the setting value in a case where the count rate caused by light emitted from the light emission unit is lowered below the first light intensity-count rate information. a radiation detection unit (11, 111) which includes a radiation detection element (11a, 111a), the radiation detection element (11a, 111a) sensing radiation and light, and emitting light of a predetermined light emission wavelength when receiving the radiation or the light; a light emission unit for emitting light of a wavelength different from the predetermined light emission wavelength; a wavelength selection unit which is configured to be set in a first mode, wherein in the first mode the wavelength selection unit passes the light of the predetermined light emission wavelength, and blocks the light from the light emission unit; an optical transmission line (15a - 15f, 151a) for transmitting the light from the light emission unit to the radiation detection unit (11, 111), and for transmitting the light from the radiation detection unit (11, 111) to the wavelength selection unit; a light detection unit for converting the light passing through the wavelength selection unit into an electric pulse; and a control unit for measuring a count rate of the electric pulse, the radiation monitor (100, 100A) is characterized in that the control unit is configured to determine whether at least the light emission unit is degraded on the basis of the count rate and the light intensity of the light emission unit, and the control unit is further configured to include a memory unit for storing first light intensity-count rate information indicating a relation between the light intensity of the light emission unit when it is already known that the radiation monitor (100, 100A) functions normally and the count rate in a state where the wavelength selection unit is set to the first mode, and the control unit is further configured to perform maintenance of the radiation monitor (100, 100A), wherein the wavelength selection unit is set to the first mode wherein during maintenance a dose rate of radiation incident on the radiation detection element (11a, 111a) is at a level of natural radiation and wherein the control unit during maintenance is configured to determine, on the basis of a setting value of a predetermined light intensity, that the light emission unit is degraded when an actual light intensity of the light emission unit is smaller than the setting value in a case where the count rate caused by light emitted from the light emission unit is lowered below the first light intensity-count rate information.