To enable to display a scale indicative of a reference length on a screen of a display at an accurate length even when exchanging the lens in an image pickup apparatus to any lens.In a case of mounting an exchange lens (4F, 4Z) to an image pickup apparatus main body (3) and displaying an image of an object picked up by an image sensor (2) and a scale (5) indicative of a reference length in juxtaposition on a screen of a display (6), a previously measured optical magnification or an actual magnification is outputted from optical magnification outputting means (8) provided to the exchange lens (4F, 4Z) is outputted, an actual magnification on the screen of the display (6) is calculated based on the optical magnification, the size for the image sensor and the screen size, and the size of the scale on the screen is set based on the actual magnification and the reference length of the scale by a scale setting means (17).