1. An obstacle detection device for detecting an obstacle around a vehicle from a spectral image obtained by an imaging unit mounted on the vehicle, the obstacle detection device comprising: an image storage unit for storing an image provided by the imaging unit; a flow extraction unit for extracting a flow of a pixel from images stored at two different times in the image storage unit, the two different times being a current time and an earlier time than the current time; a vehicle movement amount estimation unit for estimating an amount of movement of the vehicle in a real world based on the flow; a road plane estimation unit for estimating a position of a road plane based on the flow and the amount of movement of the vehicle; an obstacle candidate point relating unit relating pixels in images at the two different times by using a pixel in an image at the earlier time as a reference, by assuming that the pixel corresponds to the road plane in the real world, and by extracting a corresponding pixel at the current time based on the amount of movement of the vehicle; a non-matching range identifying unit for comparing, pixel by pixel, partial images having centers at obstacle candidate points related between the pixels in the images at the two different times, and identifying a range in which pixel values are different as a non-matching range; and an obstacle determining unit for comparing the pixel values in spectral images at the two different times corresponding to the non-matching range, and determining whether the obstacle candidate point comprises a point corresponding to a shadow on the road plane or a point corresponding to an actual obstacle.