The present invention allows to estimate a continuous surface that defines a boundary surface of an object using image data, even if the image data was acquired by an imaging method in which image information on the boundary surface of the object is limited. An image processing apparatus according to the present invention includes an estimating unit that, using the brightness gradient of a voxel group constituting volume data which represents a reconstructed image, estimates a continuous surface which defines a boundary surface of an object corresponding to the volume data.
