A radiation detection apparatus (10A, 10B, 10C) comprising: a wavelength selecting unit that is configured to allow no light other than light having a wavelength selected as a wavelength when a light emission of gas excited by an alpha ray is occurred to pass through the wavelength selecting unit; a first polarization selecting unit that is configured to receive the light from the wavelength selecting unit, to select a polarization direction, and to allow a light of the selected polarization directions to pass through the first polarization selecting unit, wherein the first polarization selecting unit is configured to select a first polarization direction and a second polarization direction; an optical system (13, 13a; 13b1, 13b2) that is configured to form an image of the light passed through the wavelength selecting unit and the first polarization selecting unit, wherein the optical system (13, 13a; 13b1, 13b2) is configured to form a first image in a first case where the first polarization selecting unit selects the first polarization direction and a second image in a second case where the first polarization selecting unit selects the second polarization direction; a photon detecting unit (14, 14a) that is configured to observe the image formed by the optical system (13, 13a; 13b1, 13b2), and to detect the photon in whole range of the image wherein the photon detecting unit (14, 14a) is configured to detect the photon in whole range of the first image and the photon in whole range of the second image; and a counting unit that is configured to count the photon detected by the photon detecting unit (14, 14a), to identify the photon derived from the light emission of gas excited by the alpha ray based on a value obtained from a result of counting the detected photon, to count the identified photon, and to calculate the number of the alpha ray based on a result of counting the identified photon, wherein the counting unit is configured to use a difference value as the value obtained from the result of counting the detected photon, the difference value being between the number of the detected photon in whole range of the first image and the number of the detected photon in whole range of the second image. a wavelength selecting unit that is configured to allow no light other than light having a wavelength selected as a wavelength when a light emission of gas excited by an alpha ray is occurred to pass through the wavelength selecting unit; a first polarization selecting unit that is configured to receive the light from the wavelength selecting unit, to select a polarization direction, and to allow a light of the selected polarization directions to pass through the first polarization selecting unit, wherein the first polarization selecting unit is configured to select a first polarization direction and a second polarization direction; an optical system (13, 13a; 13b1, 13b2) that is configured to form an image of the light passed through the wavelength selecting unit and the first polarization selecting unit, wherein the optical system (13, 13a; 13b1, 13b2) is configured to form a first image in a first case where the first polarization selecting unit selects the first polarization direction and a second image in a second case where the first polarization selecting unit selects the second polarization direction; a photon detecting unit (14, 14a) that is configured to observe the image formed by the optical system (13, 13a; 13b1, 13b2), and to detect the photon in whole range of the image wherein the photon detecting unit (14, 14a) is configured to detect the photon in whole range of the first image and the photon in whole range of the second image; and a counting unit that is configured to count the photon detected by the photon detecting unit (14, 14a), to identify the photon derived from the light emission of gas excited by the alpha ray based on a value obtained from a result of counting the detected photon, to count the identified photon, and to calculate the number of the alpha ray based on a result of counting the identified photon, wherein the counting unit is configured to use a difference value as the value obtained from the result of counting the detected photon, the difference value being between the number of the detected photon in whole range of the first image and the number of the detected photon in whole range of the second image.