1. An image processing method for performing registration between a first medical image and a second medical image both being three-dimensional images, the image processing method comprising: extracting a first vascular image in the first medical image and a second vascular image in the second medical image; detecting at least one vascular portion structure including a plurality of vascular portions close or connected to each other with respect to each of the first and second vascular images extracted; for each of a plurality of combinations of the at least one vascular portion structure in the first vascular image and the at least one vascular portion structure in the second vascular image, performing coordinate transformation on at least one of the first and second vascular images such that a deviation between the vascular portion structures becomes small, and calculating similarity in a prescribed area including the vascular portion structures between the first and second vascular images; and performing coordinate transformation for the registration using the coordinate transformation highest in the calculated similarity.