To provide a mirror angle detection device capable of stable mirror angle control by removing non-linearity of a mirror angle detection characteristic caused by stray light from a photodetector.

This mirror angle detection device comprises LD1 which is a light source, a movable part having at least a mirror 6, a beam splitter 3 for changing an optical path of reflected light from the movable part, the photodetector 7 for receiving the reflected light from the movable part and detecting the inclination quantity of the mirror 6, and at least one condensing lens 5 provided between the photodetector 7 and the movable part. In the device, a shielding plate 4 for preventing the stray light from the photodetector 7, a peripheral member or an optical member edge part from arriving at the photodetector 7 is arranged between the condensing lens 5 and the beam splitter 3. An aperture 4a for transmitting light is formed on the center part of the shielding plate 4, and the part other than the aperture 4a has a shape for shielding the light, and the shape of aperture 4a is a shape corresponding to a variable angle range of the mirror 6.