1. An image processing apparatus for integrating a plurality of shape data representing a shape of an object, the apparatus comprising: a first calculation unit configured to receive 3-D positions representing voxels, each of which is a unit for dividing a 3-D space, and the shape data and to perform a calculation of first signed distances, each of which is a signed distance between the 3-D position and the shape data, on the shape data; a second calculation unit configured to classify the voxels into a voxel that is located in a near-surface of the shape, a voxel that is located outside of the shape, or a voxel that cannot be determined to be located outside of the shape or in the near-surface based on the 3-D position representing each of the voxels and one or both of any one of the plurality of shape data and the first signed distances; a third calculation unit configured to estimate whether the voxel that cannot be determined to be located outside of the shape or in the near-surface is located inside of the shape or outside of the shape based on one or both of the plurality of shape data and the first signed distances; and a fourth calculation unit configured to calculate the 3-D positions of the voxels that are classified to be located in the near-surface and the 3-D positions of the voxels that are located in a boundary portion between the voxels that are classified or estimated to be located inside the shape and the voxels that are classified or estimated to be located outside the shape, as surface positions of the shape, wherein the third calculation unit performs a Bayes estimation on the shape data to determine whether the voxel is located inside or outside the shape based on an average value of a probability distribution of the Bayes estimation.