1. A keypoint extraction device for extracting a keypoint which is robust to image change, comprising: a hardware processor and a memory storing instructions executed by the processor, wherein the instructions are configured to be executed by the hardware processor, the instructions comprise: a keypoint extractor configured to extract a corner region in an image as a keypoint candidate region, and extract the keypoint from the keypoint candidate region; and a keypoint postprocessor configured to generate a keypoint descriptor with respect to the extracted keypoint; wherein the extracting of the corner region in the image as the keypoint candidate region, comprise: calculating a difference between an average value of pixel values in a window before the window is moved in the image and an average value of the pixel values in the window after the window is moved in the image, in the keypoint extraction device; and determining the corner region using a change amount of the average value in a direction in which the difference of the average value is the greatest and a change amount of the average value in a direction in which the difference of the average value is the smallest, in the keypoint extraction device.