1. A recognition object detecting apparatus, comprising: means for generating image data representing a taken image; and means for detecting a recognition object from the image represented by the image data, wherein the generating means has a characteristic in which a relation between luminance and output pixel values varies depending on a luminance range, and the detecting means binarizes the output pixel values of the image represented by the image data by using a plurality of threshold values to generate a plurality of binary images, and detects the recognition object based on the plurality of binary images, the generating means takes an image of a scene ahead of an own vehicle, the detecting means detects tail lights of a leading vehicle or headlights of an oncoming vehicle as the recognition object, and the detecting means binarizes the output pixel values of the image represented by the image data by using four threshold values including a first threshold value and a second threshold value which are set to respectively detect tail lights of a leading vehicle positioned at a long distance from the own vehicle and headlights of an oncoming vehicle positioned at a long distance from the own vehicle, and a third threshold value and a fourth threshold value which are set to respectively detect tail lights of a leading vehicle positioned at a short distance from the own vehicle and headlights of an oncoming vehicle positioned at a short distance from the own vehicle, to generate four of the binary images, and detects the recognition object based on the four of the binary images.