1. An image processing apparatus comprising: an acquisition unit which acquires a first image and a second image; an image detection unit which detects an image corresponding to a pattern having a first size from a first detection region in the first image acquired by said acquisition unit and from a second detection region different from the first detection region in the first image; and a determination unit which determines whether or not to detect an image corresponding to the pattern having the first size from the first detection region in the second image acquired by said acquisition unit and whether or not to detect an image corresponding to the pattern having the first size from the second detection region in the second image, in accordance with detection results of detecting an image corresponding to the pattern having the first size from the first detection region in the first image and from the second detection region in the first image.