In an error diffusion process, a random number acquiring unit acquires a random number included in a first random number range that depends on the gradation value of the target pixel data, in a case that the gradation value of the target pixel data is in a first range. The first correcting unit corrects the gradation value of the target pixel data into a first corrected gradation value by using the random number. The dot value setting unit sets a dot value of the target pixel data to either a first dot value or a second dot value. The first random number range corresponding to the gradation value smaller than the second threshold value includes a specific random number such that the first correcting unit corrects the gradation value into the first corrected gradation value greater than the second threshold value by using the specific random number.