1. An image processing apparatus, comprising: an extraction unit operable to extract a plurality of feature points from an inputted image; a feature amount computation unit configured to compute respective feature amounts for the plurality of feature points; an estimation unit configured to estimate respective fluctuation feature amounts for the plurality of feature points in a case where a prescribed plurality of alterations is applied to the inputted image; and a retrieval unit operable to retrieve an image similar to the inputted image from among a plurality of stored images, based on the fluctuation feature amounts estimated by said estimation unit.