To stably and precisely extract an object region corresponding to an object to be imaged.

A region extracting device 1 comprises: a smoothing part 7 for generating a smoothed image obtained by smoothing an inputted image; an initial region detecting part 8 for detecting an initial region including at least a part of the object to be imaged from the smoothed image, based on a smoothed pixel value held by each pixel in the smoothed image; a region deforming part 9 for discriminating whether or not the pixel close to a contour is an object region pixel, which constitutes the object region, based on the smoothed pixel value held in the pixel close to the contour in an initial region, deforming at least the size or shape of the initial region in response to the discrimination result, and forming the deformed regions; and a region integrating part 10 for detecting an adjacent region group which is formed with the adjacent deformed regions among the respective deformed regions and forming the object region by integrating the adjacent deformed regions.