1. An image processing apparatus comprising: an examination image obtaining unit configured to obtain an examination image which is an image of an examination subject; a shift-invariant feature quantity calculating unit configured to calculate, for each pixel, a shift-invariant feature quantity which is represented by predetermined base vectors, from the examination image obtained by the examination image obtaining unit; a selecting unit configured to select, on the examination image, a pixel having a matching degree lower than or equal to a predetermined threshold, between (i) a relative positional relationship of classes in a plurality of normal images each of which does not include a lesion site and (ii) a relative positional relationship of the classes to which a plurality of shift-invariant feature quantities respectively belong in the examination image, the classes being obtained by clustering the plurality of shift-invariant feature quantities which are: calculated from a plurality of pixels included in the normal images; and represented by the predetermined base vectors; and an output unit configured to output a result of the selection performed by the selecting unit.