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: brightness distribution acquisition means for acquiring a brightness distribution in the thickness direction of a blood vessel wall in a tomogram with regard to the blood vessel, setting means for setting the tunica intima reference point and the tunica externa reference point based on the brightness distribution, extraction means for extracting pixels, with respect to each pixel in a setting range including the tunica intima reference point or the tunica externa reference point, wherein the brightness belongs to the setting range, and calculation means for calculating first boundary in the lumen side between an intravascular lumen and an end of a region formed by the pixels being extracted, calculating a second boundary in the lumen side in a region formed by the pixels being extracted based on the tunica externa reference point, and calculating a distance between the first boundary and the second boundary by interpolating the composite thickness of the tunica intima and the tunica media.