1. A resolution converting method for converting to expand an input image having a binary image of a dot-matrix form into an output image having a binary image represented in a dot-matrix form having a resolution equal to an integral multiple of a resolution of the input image, the resolution converting method comprising: representing a density at an arbitrary position residing between pixels of the input image as an interpolating value obtained by interpolating vicinity pixels in the input image; allotting an area of a pixel of each pixel of the output image to the pixel, in a state of overlapping an area of the output image after a resolution conversion processing is completed, on an area of the input image, in a setting that the area of the output image is shifted from the area of the input image by less than a half of a pitch between output pixels in a horizontal direction and a vertical direction; integrating interpolated values of with respect to an area of a part of the input image which overlaps with the area of the pixel of each pixel of the output image in the setting that the area of the output image is shifted; normalizing a value obtained by the integrating with respect to a dimension of the area of the pixel; and determining a binary data of the pixel by comparing between the value obtained by the normalizing and a predetermined value.