Operability in measuring a Cobb angle of a vertebra image is improved and erroneous diagnosis is prevented. A vertebra image having a plurality of curved portions is displayed on the monitor screen of a display section and auxiliary lines are drawn along the axial directional upper end of a superior end vertebra and the axial directional lower end of an inferior end vertebra of each curved portion based on manipulation through an operational section. At that time, drawing of the individual auxiliary lines is permitted only in a sequence along the head-to-tail direction of the vertebra image displayed on the monitor screen. Then, an angle defined by two auxiliary lines adjoining in the drawing sequence (head-to-tail directional sequence) of the individual auxiliary lines is measured as a Cobb angle. When one of the auxiliary lines is shifted, the individual auxiliary lines are ordered in a shorter-to-longer sequence of the distance from one side equivalent to a head-side end portion of the image based on positional information of each auxiliary line after shifting. Then, an angle defined by two auxiliary lines adjoining in the ordered sequence is measured again as a Cobb angle.
