An image processing apparatus is provided with a halftone frequency determining section for determining a halftone frequency of an input image data including a plurality of color components. The halftone frequency determining section includes: a threshold value setting section for setting a threshold value for a segment block that is consisting of a plurality of pixels, the threshold value being for use in binarization; a threshold value adjusting section for adjusting the threshold value according to input image data in order to remove an influence of an unnecessary color component; a maximum transition number averaging section for averaging maximum transition numbers of the binary data according to the adjusted threshold value in order to obtain a maximum transition number average; and a halftone frequency estimating section for estimating the halftone frequency from the maximum transition number average. With this, it is possible to determine the halftone frequency accurately.