A binarizing method for binarizing an original image of an electrode pad on a substrate. The method includes the steps of processing an electrode image to generate a binary image of the electrode by binarzing the original image with a first threshold value, calculating a contact trace area to calculate the 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, processing a contact trace image to generate the contact trace area binary image by binarizing the original image of the area extracted by the contact trace area extracting unit with a second threshold value different from the first threshold value, and synthesizing the images to synthesize the binary image of the electrode and the contact trace area binary image by taking a logical sum on a corresponding pixel to pixel basis.