1. A method of positioning an object on which a mark is formed, the mark including a plurality of elements arranged at predetermined intervals in a direction, the method comprising steps of: forming an image of the mark on the object on a sensor; performing a plurality of zero phase filterings having each of different orders for an image signal obtained by the sensor to obtain each of filtered signals corresponding to the different orders; calculating a plurality of intervals between each of adjacent element pairs of the plurality of elements from each of the filtered signals; determining an order of the zero phase filtering by which a variation of the calculated intervals is below a predetermined threshold; detecting a position of the mark on the object from the filtered signal obtained by the zero phase filtering with the determined order; and positioning the object based on the detected position.