A vehicle exterior environment detection apparatus includes an image width calculator, a predicted distance calculator, and a relative distance calculator. The image width calculator calculates a first image width of a target vehicle on the basis of a first image. The predicted distance calculator calculates a first predicted distance to the target vehicle on the basis of the first image width. The relative distance calculator calculates a first reliability of the first image width, and, when the first reliability is higher than a predetermined threshold, calculates a first real width of the target vehicle on the basis of the first image width and the first predicted distance, updates a smoothed real width by performing smoothing processing on the basis of the first real width, and calculates a first distance to the target vehicle on the basis of the smoothed real width and the first image width.