PURPOSE: To improve detection sensitivity to a specific wavelength by providing a transparent membrane on a light-receiving plane of a transparent light- receiving plate, with a refraction factor n which is larger than that of the transparent light-receiving plate and thickness d and selecting an odd integer to a wavelength λ to satisfy specific condition to the wavelength λ.

CONSTITUTION: A conductive transparent membrane 7 is attached and formed onto an outer surface (a light-receiving plane) of a transparent light-receiving plate 2 of a light-receiving tube. A leak current thus flows to the ground from such as a photoelectric conversion membrane 3. Therefore, the light receiving plane is not charged so that decrease of light-receiving sensitivity due to attachment of dust in air to the light receiving plane is avoided. Detection sensitivity to a specific wavelength λ is improved by satisfying the condition n.d=(λ/4).m, where (n) is a refraction factor of the membrane 7 which is larger than that of the light receiving plane, (d) thickness of the membrane 7 and m an odd integer to the wavelength λ.