1. An image processing apparatus comprising: an image acquisition unit configured to acquire a three-dimensional image captured by imaging a subject using a first imaging device and a two-dimensional image captured by imaging a surface of the subject using a second imaging device; an information acquisition unit configured to acquire information representing a surface position of the subject from the three-dimensional image; a first generation unit configured to generate a projection image in a case where seeing the three-dimensional image from a viewpoint of the second imaging device, based on the information representing the surface position; and an alignment unit configured to align the three-dimensional image with the two-dimensional image with respect to the subject using the projection image and the two-dimensional image.