1. An imaging apparatus, comprising: a quantization part that quantizes an image of M gray levels into an N-level dot image (M>N) by performing an error diffusion process on each pixel using N-1 threshold values, wherein said M gray levels are divided into N-1 sections and the threshold value of at least one section varies depending on input information on a target pixel; a neighboring pixel information detection part that detects information on a neighboring pixel of said target pixel; and a threshold value determination part that determines said threshold value used by said quantization part for said target pixel by referring to said information on the neighboring pixel.