An image processing apparatus includes: a first circuit which calculates values f(RPi), f(GPi) and f(BPi) by applying a function f(x) to an R grayscale value RPi, a G grayscale value GPi and a B grayscale value BPi of each pixel i of a first image; a second circuit which calculates values f(RQi), f(GQi) and f(BQi) by applying the function f(x) to an R grayscale value RQi, a G grayscale value GQi and a B grayscale value BQi of each pixel i of a second image; and a similarity calculation circuit which calculates a degree of similarity between the first and second images depending on |f(RPi)−f(RQi)|, |f(GPi)−f(GQi)| and |f(BPi)−f(BQi)| associated with each pixel i of the first and second images. The function f(x) is a convex function monotonically non-decreasing in the domain of definition.