A roundness measuring apparatus includes a stylus stocker and a controller. The stylus stocker stores plural types of styli prepared corresponding to shapes of measurement sites of a measurement target object. The stylus stocker can store styli in such a manner that each stylus can be held thereon and taken out thereof. The stylus stocker is provided outside a measurement region, which is determined on the basis of the operation range of a turntable and a detector driving mechanism. The detector driving mechanism can move the detector to the outside of the measurement region. When a measurement command is given, the controller carries out roundness measurement of the object while controlling the turntable and the detector driving mechanism. When a stylus replacement command is given, the controller carries out stylus replacement operation between a detector main unit and the stylus stocker while controlling the detector driving mechanism.