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; a threshold obtaining section which obtains a new threshold corresponding to each pixel of the input image based on a threshold of each cell composing the threshold matrix, wherein the screen processing section compares the pixel value of each pixel of the input image with the new threshold corresponding to each pixel of the input image obtained by the threshold obtaining section and generates a multivalue output image.