1. An image processing apparatus that reconstructs a multilevel image from a halftone image obtained by subjecting previous multilevel image data to halftone processing employing a threshold matrix, the image processing apparatus comprising: a restoration unit that accumulates significant dot values within a reference window that encompasses a pixel of interest in the halftone image, and that restores a multilevel pixel value that corresponds to the pixel of interest by referring to a conversion table based on the accumulated dot values, wherein a height of the reference window is determined such that a product of a width of the threshold matrix, a number of tones in the halftone image and the height of the reference window becomes a minimum value that exceeds the number of tones in the multilevel image, and the width of the reference window is set to the width of the threshold matrix.