1. An image processing method comprising: a first process for, when a block image consisting of a plurality of pixels is defined as an objective image, calculating a parameter representing whether an effective density pixel having a predetermined density value exists in the objective image; a second process for, when one of pixels in the objective image is defined as an interest pixel and a pixel neighboring downstream side in a predetermined first direction with respect to the interest pixel is defined as a neighboring pixel, converting the objective image to an update image on lines from which one line is reduced in the first direction, by defining the interest pixel as the effective density pixel when a first condition on which the interest pixel is an effective density pixel and a second condition on which at least one neighboring pixel with respect to the interest pixel is an effective density pixel are satisfied, and by defining the interest pixel as an ineffective density pixel when either the first condition or the second condition is satisfied, as to each interest pixel having at least one neighboring pixel in the objective image; a third processing for calculating a parameter representing whether an effective density pixel exists in the update image; and a fourth processing for, when the second and third processes are repeated with respect to the update image defined as the objective image after the third process so that the objective image is lined on one line in the first direction, calculating a number of continuous pixels of the effective density pixels being spatially continuing in the block image on the basis of each parameter calculated in the first and the third processes.