A reference designation point setting unit sets a plurality of reference designation points. A reference calculation unit calculates one of a reference line and a reference plane used in a measurement mode indicated by measurement mode information, on the basis of the plurality of reference designation points. A measurement point setting unit sets a measurement point. A reference point calculation unit calculates a plurality of reference points leading to higher reliability of a measurement result instead of a plurality of reference designation points, on the basis of the image in which the measurement point is set, the plurality of reference designation points, the measurement mode, and the measurement point. The reference calculation unit calculates one of a new reference line and a new reference plane on the basis of the plurality of reference points.