1. An image processing apparatus comprising: a first storage unit which previously stores a desired volume three-dimensional image; a second storage unit which stores a sequentially updated two-dimensional image; an alignment unit which aligns the three-dimensional image stored in the first storage unit with the two-dimensional image stored in the second storage unit; a position acquisition unit which acquires a position of a device; and a computing unit which calculates the position of the device in the three-dimensional image.