An input gradation converting circuit has a gradation conversion table in which M-bit intermediate gradation data are correlated with each of N-bit input gradation data, and is operable to convert each of the input gradation data into the M-bit intermediate gradation data by using the gradation conversion table and output the converted M-bit intermediate gradation data. A diffusion gradation converting circuit is operable to convert each of the intermediate gradation data outputted from the input gradation converting circuit into N-bit output gradation data and output the converted N-bit output gradation data. The diffusion gradation converting circuit acquires a uniform random number Rnd in the range of −SFT/2 to SFT/2 and outputs [X+Rnd+0.5] as the output gradation data for each of the intermediate gradation data in which X obtained by dividing a value of the intermediate gradation data by 2M·N and a specified value SFT satisfy a conditional expression “SFT/2≦[X]≦2N−1−SFT/2” (where [ ] is a Gauss symbol). The diffusion gradation converting circuit acquires a uniform random number Rnd′ in the range of 0≦[X+Rnd′+05]≦2N−1 and outputs [X+Rnd′+0.5] as the output gradation data for each of the intermediate gradation data in which the X and the specified value SFT do not satisfy the conditional expression.