1. A method for setting pattern-searching condition for specifying a desired pattern on a sample, comprising the steps of: acquiring a first image obtained by scanning a charged particle beam at a first magnification; acquiring at a second magnification lower than the first magnification a second image which contains a pattern image displayed on the first image; making a size of the first image coincident with a size of a third image which cut out a portion of the second image, and setting the second magnification as a magnification in acquiring an image to be searched in the pattern searching in the case where a correlation value between the both is equal to or higher than the predetermined value.