A method of displaying a medical image, the method comprising: generating, by an image processor, first images showing a region of interest (ROI) by using echo signals corresponding to ultrasound signals transmitted toward the ROI; selecting, by a controller, second images showing the ROI from among prestored images, the second images being generated at a time point in the past; and displaying, by a display, the first images and the second images on a screen; wherein the displaying the first images and the second images on the screen comprises: displaying first thumbnail images corresponding to the first images in a region of the screen and second thumbnail images corresponding to the second images in another region of the screen; receiving a user selection of a thumbnail image among the first thumbnail images, and selecting, by the controller, an image that is most similar to the selected first thumbnail image from among the second thumbnail images; or receiving a user selection of a thumbnail image among the second thumbnail images, and selecting, by the controller, an image that is most similar to the selected second thumbnail image from among the first thumbnail images; and displaying the selected first and second thumbnail images as their enlarged version in regions of the screen other than the regions where the first and second thumbnail images are displayed. generating, by an image processor, first images showing a region of interest (ROI) by using echo signals corresponding to ultrasound signals transmitted toward the ROI; selecting, by a controller, second images showing the ROI from among prestored images, the second images being generated at a time point in the past; and displaying, by a display, the first images and the second images on a screen; wherein the displaying the first images and the second images on the screen comprises: displaying first thumbnail images corresponding to the first images in a region of the screen and second thumbnail images corresponding to the second images in another region of the screen; receiving a user selection of a thumbnail image among the first thumbnail images, and selecting, by the controller, an image that is most similar to the selected first thumbnail image from among the second thumbnail images; or receiving a user selection of a thumbnail image among the second thumbnail images, and selecting, by the controller, an image that is most similar to the selected second thumbnail image from among the first thumbnail images; and displaying the selected first and second thumbnail images as their enlarged version in regions of the screen other than the regions where the first and second thumbnail images are displayed. displaying first thumbnail images corresponding to the first images in a region of the screen and second thumbnail images corresponding to the second images in another region of the screen; receiving a user selection of a thumbnail image among the first thumbnail images, and selecting, by the controller, an image that is most similar to the selected first thumbnail image from among the second thumbnail images; or receiving a user selection of a thumbnail image among the second thumbnail images, and selecting, by the controller, an image that is most similar to the selected second thumbnail image from among the first thumbnail images; and displaying the selected first and second thumbnail images as their enlarged version in regions of the screen other than the regions where the first and second thumbnail images are displayed. <EOS>