A method of displaying a diagnostic image is provided. The method includes setting an examination target of a subject, dividing the examination target into a plurality of examination regions, detecting whether a signal region indicated by a signal transmitted from a probe collides with each of the plurality of examination regions, calculating a collision frequency of each of the plurality of examination regions, based on a result of the detecting, and displaying a diagnostic image that indicates a sectional view of the subject and includes information indicating a degree of the collision frequency of each of the plurality of examination regions included in the sectional view, based on the calculation result.