1. An obstacle detection device, comprising: a storage unit at least storing an image; a quasi-vertical edge detection module coupled to the storage unit to receive an input image and perform a quasi-vertical edge detection procedure on the input image to obtain a plurality of quasi-vertical edges; an intersection determination module determining whether the quasi-vertical edges intersect a predetermined virtual horizontal; a position determination module determining whether an end of one of the quasi-vertical edges intersecting the virtual horizontal is located in a detection zone if one of the quasi-vertical edges intersects the virtual horizontal; and an obstacle detection module determining that an obstacle exists and issuing a warning if one end of one of the quasi-vertical edges intersecting the virtual horizontal is located in the detection zone.