1. An image processing apparatus, comprising: an image input device configured to read an original image and to obtain image data from the original image by selecting one of plural types of image reading modes of the image input device, wherein the image reading modes of the image input device include a first image reading mode in which the image data of the original image is obtained by moving a sensor of the image input device relative to the original image and a second image reading mode in which the image data of the original image is obtained by moving the original image relative to the sensor; and an information extracting mechanism configured to change a threshold value used for determining existence of dots according to the image reading mode selected by the image input device, and to extract specific information included in the image data obtained by the image input device using the threshold value that is changed according to the image reading mode selected by the image input device, wherein when the first image reading mode is selected, the threshold value used for determining existence of dots is changed to have a smaller value than the threshold value used when the second image reading mode is selected so as to suppress omission of detection of the dots.