1. An image processing device comprising: an extractor configured to extract a region of interest which satisfies a specified condition from a first image frame; a size decision unit configured to decide an image size according to a size of the region of interest extracted by the extractor; and an image converter configured to change the size of an image frame to be processed according to the image size decided by the size decision unit to generate a transformed image frame, wherein the extractor extracts a region of interest which satisfies the specified condition from the transformed image frame wherein the image converter changes the size of a second image frame obtained after the first image frame according to the image size decided by the size decision unit to generate the transformed image frame, and the extractor extracts the region of interest from the transformed image frame.