1. A radiological image area extracting apparatus for extracting an area of interest of an object under examination from a radiological image picked up, the apparatus comprising: a low-frequency image generating unit for generating a low-frequency image from the radiological image by a frequency filter process lower than a spatial frequency inside an object area; a luminance variation image generating unit for generating a luminance variation image by detecting luminance variations of the low-frequency image; and a boundary selecting unit for selecting boundary pixels located on a boundary of the area of interest based on pixel values of the luminance variation image and the low-frequency image.