1. An alignment method, comprising the steps of: taking an image of a first portion of a wafer by means of an image sensor; memorizing image data related to a pattern of the first portion and obtained by the image-taking by the image sensor; moving the wafer relative to the image sensor; taking, by means of the image sensor, an image of a second portion of the wafer, which second portion is closer to the first portion than is a third portion of the wafer; predicting the position of a pattern of the third portion on the bases of (i) the memorized image data related to the pattern of the first portion and (ii) image data related to a pattern of the second portion and obtained by the image-taking by the image sensor; moving the wafer relative to the image sensor on the basis of the prediction; taking an image of the third portion of the wafer by means of the image sensor; and detecting an error of the wafer in a rotational direction, on the basis of (i) the memorized image data related to the pattern of the first portion and (ii) image data related to the third portion and obtained by the image-taking by the image sensor.