PURPOSE:To reduce an obstacle detector for a vehicle in size by using the same space for both light emission and photodetection, and to prevent malfunction by subtracting a signal of reflected light from a signal outputted from a photodetecting element. CONSTITUTION:Laser light is emitted to the front of a vehicle through a lens 12 and part of it is reflected by the surface of the lens 12 and made incident to a photodetecting element 13 and a photodetecting element 14 for monitoring. The laser light is diverged at a specific angle because a laser diode 11 is closer to the lens 12 than to the focus of the lens 12. Reflected light reflected by an obstacle at the front position is converged again by the lens 12 and guided to the photodetecting element 13. The photodetecting element 13 converts the light into an electric signal, which is inputted to a photodetecting circuit and processed properly to obtain a speed. This constitution reduces the side of the device and a means which subtracts the output signal of the photodetecting element for monitoring from the output signal of the photodetecting element to prevent malfunction.
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