First adding circuit (101) sums an input image signal of a target pixel and a value that is derived by multiplying respective display errors of three pixels before the target pixel by one line by weighting factors. Gradation candidate converting means (102) converts the gradation of the summed image signal to a plurality of available gradation candidates that are convertible when an error of the immediate left pixel is added. Delaying circuit (103) delays the summed image signal. Second adding circuit (104) sums the delayed image signal and a value that is derived by multiplying the error generated in the immediate left pixel by a weighting factor. Gradation selecting means (105) selects a gradation closest to the gradation of the image signal to which the error is added from a plurality of gradation candidates, and outputs it as an image signal of the target pixel. Differencing circuit (106) determines a display error of the target pixel.