To make it possible to detect only α-ray even under the environment where the α-ray and γ-ray are mixed by converting the wavelength band of fluorescence, and emitting wavelength converted fluorescence to the wavelength band of the high sensitivity wavelength band side of a pin photodiode.

When α-ray and γ-ray are externally incident to the scintillator 2 of ZnS (Ag) of α-ray measuring instrument 1, fluorescence responsive to the incident amount of the alpha-ray is emitted. The fluorescence is emitted as wavelength converted fluorescence via fluorescent glass 3 and pin photodiode 4. The pulse signal SP emitted from the diode 4 is amplified by an amplifier 5, noise is removed by comparator 6, and an output signal NSP is output to a counter 7. The counter 7 counts corresponding to the wavelength converted fluorescence based on the signal NSP, calculates the counted number corresponding only to the incident amount of the alpha-ray, and outputs the α-ray counted data DC to a process controller 9. As a result, the controller 9 variously processes based on the data DC and displays the result on a display 8.