1. An image processing apparatus comprising: a conversion processing unit that extracts a gradation feature value from inside an input image when the input image is acquired; an analysis processing unit that quantizes the input image in two levels based on the gradation feature value in the input image, a first threshold value smaller than a median, and a second threshold value greater than the median by replacing areas of the input image where the gradation feature value is smaller than the first threshold value and where the gradation feature value is greater than the second threshold value with a first pixel value and replacing remaining areas of the input image with a second pixel value; a storing unit that stores dictionary data including a feature of a plurality of face images quantized by the analysis processing unit; and a face detection processing unit that performs face detection on a detection target image by using the dictionary data read from the storing unit, the detection target data being quantized by the analysis processing unit, wherein the conversion processing unit performs frequency conversion on the input image and extracts a magnitude of a frequency component as the gradation feature value, and wherein the analysis processing unit quantizes the input image in two levels based on an absolute value of the gradient feature value in the input image and the second threshold by replacing an area of the input image where the gradient feature value has an absolute value greater than the second threshold with the first pixel value and replacing remaining areas of the input image with the second pixel value.