A blood vessel image processing apparatus comprising: a storage device storing three-dimensional blood vessel data; a center line specifying unit configured to specify a center line of a blood vessel based on three-dimensional blood vessel data showing a three-dimensional shape of the blood vessel; a protruding portion center line specifying unit configured to specify a center line of a protruding portion of the blood vessel from the center line specified by the center line specifying unit; a branched center line specifying unit configured to specify a center line branched from the protruding portion center line specified by the protruding portion center line specifying unit; a conversion unit configured to define, for the three-dimensional blood vessel data of the blood vessel corresponding to the branched center line specified by the branched center line specifying unit, a cylindrical coordinate system using, as coordinate axes, a blood vessel circumferential direction about the center line, a blood vessel radial direction from the center line, and a center line direction, thereby converting the blood vessel data in the cylindrical coordinate system into blood vessel data in an orthogonal coordinate system; an interpolation unit configured to use, on the blood vessel data converted into the orthogonal coordinate system by the conversion unit, a coordinate value of the coordinate axis in the radial direction at a first coordinate value of the coordinate axis in the 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; and an inverse conversion unit configured to convert, after interpolation by the interpolation unit, the blood vessel data in the orthogonal coordinate system into blood vessel data in the cylindrical coordinate system. a storage device storing three-dimensional blood vessel data; a center line specifying unit configured to specify a center line of a blood vessel based on three-dimensional blood vessel data showing a three-dimensional shape of the blood vessel; a protruding portion center line specifying unit configured to specify a center line of a protruding portion of the blood vessel from the center line specified by the center line specifying unit; a branched center line specifying unit configured to specify a center line branched from the protruding portion center line specified by the protruding portion center line specifying unit; a conversion unit configured to define, for the three-dimensional blood vessel data of the blood vessel corresponding to the branched center line specified by the branched center line specifying unit, a cylindrical coordinate system using, as coordinate axes, a blood vessel circumferential direction about the center line, a blood vessel radial direction from the center line, and a center line direction, thereby converting the blood vessel data in the cylindrical coordinate system into blood vessel data in an orthogonal coordinate system; an interpolation unit configured to use, on the blood vessel data converted into the orthogonal coordinate system by the conversion unit, a coordinate value of the coordinate axis in the radial direction at a first coordinate value of the coordinate axis in the 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; and an inverse conversion unit configured to convert, after interpolation by the interpolation unit, the blood vessel data in the orthogonal coordinate system into blood vessel data in the cylindrical coordinate system.