An image-processing apparatus comprising: object-defining means configured to define an occluding object that occludes a target object to be observed; image-obtaining means configured to obtain a captured image captured by an image-pickup apparatus; region-detecting means configured to detect a region of the captured image including the occluding object from the obtained captured image; image-feature-detecting means configured to detect image features of the target object from the captured image; and calculating means configured to calculate a position and/or orientation of the image-pickup apparatus that captured the captured image or a position and/or orientation of the target object in the captured image on the basis of the image features detected by the image-feature-detecting means, wherein the image-processing apparatus is configured not to take into account features from the detected region in determining the position and/or orientation of the image-pickup apparatus that captured the image or a position and/or orientation of the target object in the captured image, characterized by: first rough-position-and-orientation-obtaining means configured to obtain a first position and first orientation of the image-pickup apparatus; and second rough-position-and-orientation-obtaining means configured to obtain a second position of the occluding object or a second position and second orientation of the occluding object; wherein the object-defining means is configured to define a three-dimensional shape of the occluding object; and wherein the region-detecting means is configured to detect the region including the occluding object from the captured image on the basis of the first position and first orientation of the image-pickup apparatus obtained by the first rough-position-and-orientation-obtaining means, the second position of the occluding object or the second position and second orientation of the occluding object obtained by the second rough-position-and-orientation-obtaining means, and the three-dimensional shape of the occluding object. object-defining means configured to define an occluding object that occludes a target object to be observed; image-obtaining means configured to obtain a captured image captured by an image-pickup apparatus; region-detecting means configured to detect a region of the captured image including the occluding object from the obtained captured image; image-feature-detecting means configured to detect image features of the target object from the captured image; and calculating means configured to calculate a position and/or orientation of the image-pickup apparatus that captured the captured image or a position and/or orientation of the target object in the captured image on the basis of the image features detected by the image-feature-detecting means, wherein the image-processing apparatus is configured not to take into account features from the detected region in determining the position and/or orientation of the image-pickup apparatus that captured the image or a position and/or orientation of the target object in the captured image, characterized by: first rough-position-and-orientation-obtaining means configured to obtain a first position and first orientation of the image-pickup apparatus; and second rough-position-and-orientation-obtaining means configured to obtain a second position of the occluding object or a second position and second orientation of the occluding object; wherein the object-defining means is configured to define a three-dimensional shape of the occluding object; and wherein the region-detecting means is configured to detect the region including the occluding object from the captured image on the basis of the first position and first orientation of the image-pickup apparatus obtained by the first rough-position-and-orientation-obtaining means, the second position of the occluding object or the second position and second orientation of the occluding object obtained by the second rough-position-and-orientation-obtaining means, and the three-dimensional shape of the occluding object. first rough-position-and-orientation-obtaining means configured to obtain a first position and first orientation of the image-pickup apparatus; and second rough-position-and-orientation-obtaining means configured to obtain a second position of the occluding object or a second position and second orientation of the occluding object; wherein the object-defining means is configured to define a three-dimensional shape of the occluding object; and wherein the region-detecting means is configured to detect the region including the occluding object from the captured image on the basis of the first position and first orientation of the image-pickup apparatus obtained by the first rough-position-and-orientation-obtaining means, the second position of the occluding object or the second position and second orientation of the occluding object obtained by the second rough-position-and-orientation-obtaining means, and the three-dimensional shape of the occluding object.