When the input value becomes close to the medium dot relative density value (Den_M), the large dot threshold value (Thre_L) and medium dot threshold value (Thre_M) become close to each other. It is possible to prevent output values from being converged to the particular medium-dot output value. When the input value becomes close to the small dot relative density value (Den_S), the medium dot threshold value (Thre_M) and the small dot relative density value (Den_S) become close to each other. It is possible to prevent output values from being converged to the particular small-dot output value.