An image processing apparatus includes a setting unit, an averaging unit, a determination unit, a quantization unit, an addition unit and a detection unit. The averaging unit divides image data based on a division size and phase set by the setting unit to subject pixels included in the resultant respective division regions to an averaging processing to calculate an average value. The quantization unit configured to obtain a quantization value for each of the plurality of pixels. The addition unit adds the quantization values, that are obtained so that at least one of the division size and the phase is different from the other, to generate addition image data. The determination unit determines the quantization threshold value so that the quantization threshold value in a case where the division size is a first size is higher than the quantization threshold value in a case where the division size is a second size larger than the first size.