1. An image processing apparatus which uses a threshold matrix of a resolution higher than a resolution of an input image, the image processing apparatus comprising: a screen processing section which performs screen processing on an input image which is input; a matrix storage section which stores a threshold matrix of a resolution higher than a resolution of the input image, the threshold matrix being composed of a plurality of cells each of which has a predetermined threshold; a threshold obtaining section which obtains a new threshold adjusted to the resolution of the input image based on the threshold of each cell composing the threshold matrix, wherein the screen processing section compares a pixel value of each pixel of the input image with the obtained new threshold and generates a multivalue output image.