An image processing apparatus comprising: an approximate-surface calculator (16, 31) configured to calculate multiple approximate surfaces that approximate a pixel value of a pixel included in an examination-target region of an in-vivo image, wherein the in-vivo image is captured inside a body lumen; an approximate-surface selector (17, 17-2, 17-3) configured to select at least one approximate surface from the approximate surfaces on the basis of the relation between a pixel value of a pixel in the examination-target region and the approximate surfaces; an approximate-region setting unit (18, 18-2, 18-3) configured to set an approximate region where the pixel value of the pixel is approximated by at least the selected one of the approximate surfaces; and an abnormal-region detector configured to detect an abnormal region on the basis of a pixel value of a pixel in the approximate region and a value at the coordinates of the pixel on at least the selected one of the approximate surfaces, wherein the approximate-region setting unit (18, 18-2, 18-3) includes a candidate-region detector (181, 181-2) configured to detect a candidate region of the approximate region, and a candidate-region deforming unit (182, 182-3) configured to deform the candidate region, and the approximate-region setting unit (18, 18-2, 18-3) is configured to set the candidate region deformed by the candidate-region deforming unit (182, 182-3) as the approximate region, characterized in that the candidate-region deforming unit includes a morphological operation unit configured to perform a morphological operation on the candidate region, wherein the approximate-region setting unit (18, 18-2) is configured to set the candidate region deformed due to the morphological operation as the approximate region; or the candidate-region deforming unit includes an inclusion-region combining unit configured to combine the candidate region with an inclusion region included in the candidate region, wherein the approximate-region setting unit is configured to set the candidate region deformed by combining the inclusion region as the approximate region. an approximate-surface calculator (16, 31) configured to calculate multiple approximate surfaces that approximate a pixel value of a pixel included in an examination-target region of an in-vivo image, wherein the in-vivo image is captured inside a body lumen; an approximate-surface selector (17, 17-2, 17-3) configured to select at least one approximate surface from the approximate surfaces on the basis of the relation between a pixel value of a pixel in the examination-target region and the approximate surfaces; an approximate-region setting unit (18, 18-2, 18-3) configured to set an approximate region where the pixel value of the pixel is approximated by at least the selected one of the approximate surfaces; and an abnormal-region detector configured to detect an abnormal region on the basis of a pixel value of a pixel in the approximate region and a value at the coordinates of the pixel on at least the selected one of the approximate surfaces, the approximate-region setting unit (18, 18-2, 18-3) includes a candidate-region detector (181, 181-2) configured to detect a candidate region of the approximate region, and a candidate-region deforming unit (182, 182-3) configured to deform the candidate region, and a candidate-region detector (181, 181-2) configured to detect a candidate region of the approximate region, and a candidate-region deforming unit (182, 182-3) configured to deform the candidate region, and the approximate-region setting unit (18, 18-2, 18-3) is configured to set the candidate region deformed by the candidate-region deforming unit (182, 182-3) as the approximate region, characterized in that the candidate-region deforming unit includes a morphological operation unit configured to perform a morphological operation on the candidate region, wherein the approximate-region setting unit (18, 18-2) is configured to set the candidate region deformed due to the morphological operation as the approximate region; or the candidate-region deforming unit includes an inclusion-region combining unit configured to combine the candidate region with an inclusion region included in the candidate region, wherein the approximate-region setting unit is configured to set the candidate region deformed by combining the inclusion region as the approximate region.