PURPOSE: To obtain an output current based upon only target light in incident light by composing a photodetection system of a diffraction grating and an image pickup element in combination.

CONSTITUTION: The incident light is converted by a lens 2 into a parallel light beam, which is made incident on a flat plate type diffraction grating 3. The flat plate type diffraction grating 3 is continuous in angle of diffraction corresponding to the wavelength of the incident light, so the incident light is reflected to the image pickup element 5 at an angle which slightly differs with its wavelength. The image pickup element 5 is arranged being directed so that the light beam which is reflected at the slightly different angle can be detected; and only light having specific wavelength is made incident, pixel by pixel, and a potential corresponding to its intensity is generated. Namely, wavelength corresponding to each pixel is previously measured and then the potential generated at the pixel is detected to find the intensity of the incident light as a function of pixels of the image pickup element 5. The image pickup element 5 outputs a photocurrent from a pixel receiving only the target signal light in the incident light.