1. An image binarizing method for binarzing an original image of an electrode pad on a substrate, the method comprising the steps of: processing an electrode image to generate a binary image of a single entire electrode pad by binarizing the original image with a first threshold value; extracting a contact trace area including a portion that is predicted to be a trace of an object that contacted the electrode from the binary image of the electrode pad generated in the step of processing the electrode image, where the contract trace area is a part of the electrode; processing a contact trace image to generate a contact trace area binary image by binarizing an image of an area corresponding to the contact trace area obtained in the step for extracting the contact trace area with a second threshold value that is different from the first threshold value; and synthesizing the binary image of the single entire electrode pad and the contact trace area binary image, by taking a logical sum on a corresponding pixel to pixel basis; wherein the first threshold value in the step for processing the electrode image is a brightness between a maximal value of a low brightness and a maximal value of a high brightness in a frequency distribution of brightness of a pixel of the original image; and the second threshold value in the step for processing the contact trace image is a brightness between the first threshold value and the maximal value of the high brightness; wherein the first threshold value in the step for processing the electrode image is a brightness of minimal frequency between the maximal value of the low brightness and the maximal value of the high brightness.