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 an electrode 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; 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 electrode binary image and the contact trace area binary image.