An image area extracting device includes a low-frequency image generating unit for generating a low-frequency image from a 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 an area of interest based on pixel values of the luminance variation image and the low-frequency image. Influences of noise components or fine structural components inside a patient M are reduced by generating the low-frequency image, which facilitates recognition of a contour shape of the patient M. The boundary pixels are selected based on the low-frequency image and the luminance variation image generated from the low-frequency image. This assures highly accurate and stable area extraction.