1. An image processing apparatus, which determines, for a combined image obtained by combining pixels of a given first image and pixels of an unknown second image either translucently or non-translucently using an unknown coefficient indicating a transparency, whether at least one pixel included in the combined image is a translucently combined pixel, the apparatus comprising: a calculation unit configured to calculate, from pixel values of the combined image and pixel values of the first image of respective pixels in a predetermined area including one pixel, pixel values of an image corresponding to the second image when each of a plurality of values is used as the coefficient, and to calculate a total of differences between the pixel values of the one pixel and other pixels included in the predetermined area in the image corresponding to the second image; an estimation unit configured to estimate one of the plurality of values of the coefficient that has a total of differences, which is the smallest among totals of the differences corresponding to the plurality of values of the coefficient, as the coefficient used for the one pixel to obtain the combined image; and a determination unit configured to determine that the one pixel is a pixel obtained by translucently combining the first image and the second image when the estimated value of the coefficient is larger than a predetermined value.