1. An image processing method using an image processing device which detects a specific detecting object from an image including a plurality of two-dimensionally arranged pixels, comprising: quadratic-differentiating pixels arranged in a first direction of an image by the image processing device; determining a reference value of quadratic-differential values based on a frequency distribution of the quadratic-differential values after carrying out said quadratic-differentiating; converting a quadratic-differential value of a pixel into a predetermined value for each of the quadratic-differential values of the pixels that is not less than the reference value; accumulating data based on the predetermined value into which the quadratic differential value was converted, for the quadratic-differential values of the pixels in a second direction different from the first direction, by the image processing device; and detecting positions of the detecting object in the first direction in the image based on accumulated results by the image processing device.