A control device defines, for three-dimensional blood vessel data of a blood vessel corresponding to a branched center line, a cylindrical coordinate system, thereby converting this into blood vessel data in an orthogonal coordinate system. The control device uses a coordinate value of the coordinate axis in a radial direction at a first coordinate value of the coordinate axis in a center line direction and a coordinate value of the coordinate axis in the radial direction at a second coordinate value of the coordinate axis in the center line direction, thereby interpolating a coordinate value of the coordinate axis in the radial direction between the first coordinate value and the second coordinate value. The control device converts, after interpolation, the blood vessel data into blood vessel data in the cylindrical coordinate system.
