1. An alignment method for positioning an object on which plural marks are formed, said alignment method comprising steps of: forming an image of plural marks of the object on a sensor; forming a polynomial approximated signal corresponding to each order by performing plural types of polynomial approximation having different polynomial approximation orders for an image signal obtained by the sensor; calculating plural intervals between the plural marks using each of the polynomial approximated signal; determining an order of the polynomial approximation for alignment based on a threshold and values each of which shows a variation of calculated intervals for each of the filtered signal; detecting positions of plural marks of the object through the polynomial approximation in the determined order; and positioning the object using a detection result of the detecting step.