A computer-readble medium stored with a program causing a computer to quantize a gray level causes the computer to perform a pixel selecting process; determining a noise use rate indicating the degree to which a dither matrix noise has an influence on the quantization and an error use rate indicating the degree to which an accumulated error has an influence on the quantization and determining the noise use rate and the error use rate corresponding to the corresponding pixel depending on the gray levels of the pixels sequentially selected in the pixel selecting process; and performing the quantization on the gray levels of the pixels sequentially selected in the pixel selecting process and performing the quantization using the dither matrix noise based on the noise use rate corresponding to the corresponding pixel and using the accumulated error based on the error use rate corresponding to the corresponding pixel.