1. A preceding vehicle detection apparatus comprising: a computer; an image capture unit for capturing an image of surroundings of a subject vehicle; a preceding vehicle detection unit for detecting a preceding vehicle from the image captured by the image capture unit; and a tail lamp detection unit for detecting a pixel area having a luminance which is larger than or equal to a first threshold value on a pixel line vertically extending in the image and detecting a pixel area in each pixel line while scanning pixel lines in leftward and rightward directions from the pixel line set as a reference so as to detect areas where tail lamps of the preceding vehicle are captured from the image, wherein when it is determined, with input from the computer, that the number of pixels of the pixel area detected on each pixel line while the pixel lines on the image are being scanned in each of the leftward and rightward directions tends to decrease as the pixel lines are scanned in each of the directions, the tail lamp detection unit finishes the detection of the pixel area at a pixel line whose pixel number is more than or equal to a constant number times of a minimum value of the pixel number.