1. A medical imaging diagnostic apparatus that obtains image data from a blood vessel of an object being examined and measures the composite thickness of a tunica intima and a tunica media of the blood vessel, comprising extraction means for extracting the tunica intima and the tunica externa of the blood vessel based on the brightness information of the image data, wherein the composite thickness of the tunica intima and the tunica media of the blood vessel is measured based on the extracted two regions.