A method of displaying a virtual ruler on separate images of an object includes: dividing the object into imaging areas in a predetermined direction and obtaining the separate images corresponding to the imaging areas obtaining a first distance value for each respective separate image, as a distance from a reference point to a first side of the respective separate image, and a second distance value for each respective separate image, as a distance from the reference point to a second side of the respective separate image, the first side being opposite to the second side along the predetermined direction and displaying the virtual ruler, which indicates distance values between the first distance value and the second distance value of each respective separate image, on each of the respective separate images.
