PURPOSE: To detect the quantity of light corresponding to a discharging direction angle and an angle quickly and stably with high resolution by mounting a solid- state photodetector generating plural diffraction figures through a secondary diffraction grating and performing self-rotational scanning on the center of each diffraction figure generated on a place other than the optical axis.

CONSTITUTION: Plane wave light made incident from the left side is transmitted to an picture 7 with the distribution of light transmission strength and focused on an image formation surface 3 through a secondary diffraction grating 1 and a lens 2. When a square diffraction grating shown in the drawing (b) is used for said diffraction grating 1, primary diffraction luminous points appearing on the image formation surface 3 are four points as shown in the drawing (c). Disklike self-rotation scanning photodetector 4 are mounted on these diffraction luminous points. Each photodetector 4 has concentrically arranged light receivers 14 and electric signals obtained from respective photodetecting elements 14a1, 14a2, 14a3... which are arranged like a band-ring are outputted from an output terminal 5.