A roundness measuring apparatus comprising: a base; a turning table that can rotate around a vertical axis, a measurement target object being placed on an upper surface of the turning table; a detector that includes a stylus and a detector main unit, the stylus being brought into contact with the measurement target object, the stylus being detachably attached to the detector main unit, the detector main unit detecting displacement of the stylus and outputting the detected stylus displacement in the form of an electric signal; a detector driving mechanism that drives the detector so as to move the detector in a direction of the vertical axis and in a first direction orthogonal to the vertical axis toward and away from the turning table; a stylus stocker that stores plural types of styli, which are prepared corresponding to shapes of measurement sites of the measurement target object, the stylus stocker being provided outside a measurement region, which is determined on the basis of an operation range of the turning table and the detector driving mechanism, and configured to store the styli in such a manner that each stylus is held on the stylus stocker horizontally in the stylus stocker with a longitudinal axis of the stylus in a second direction orthogonal to the vertical axis and taken out of the stylus stocker; and a control unit which is configured to carry out measurement of roundness of the measurement target object while controlling operation of the turning table and the detector driving mechanism when a measurement command is given, wherein the detector driving mechanism is configured to be able to move the detector to the outside of the measurement region to where the stylus stocker is arranged, and the roundness measuring apparatus being configured so that when a stylus replacement command is given, the control unit carries out stylus replacement operation between the detector main unit and the stylus stocker while controlling operation of the detector driving mechanism. a base; a turning table that can rotate around a vertical axis, a measurement target object being placed on an upper surface of the turning table; a detector that includes a stylus and a detector main unit, the stylus being brought into contact with the measurement target object, the stylus being detachably attached to the detector main unit, the detector main unit detecting displacement of the stylus and outputting the detected stylus displacement in the form of an electric signal; a detector driving mechanism that drives the detector so as to move the detector in a direction of the vertical axis and in a first direction orthogonal to the vertical axis toward and away from the turning table; a stylus stocker that stores plural types of styli, which are prepared corresponding to shapes of measurement sites of the measurement target object, the stylus stocker being provided outside a measurement region, which is determined on the basis of an operation range of the turning table and the detector driving mechanism, and configured to store the styli in such a manner that each stylus is held on the stylus stocker horizontally in the stylus stocker with a longitudinal axis of the stylus in a second direction orthogonal to the vertical axis and taken out of the stylus stocker; and a control unit which is configured to carry out measurement of roundness of the measurement target object while controlling operation of the turning table and the detector driving mechanism when a measurement command is given, wherein the detector driving mechanism is configured to be able to move the detector to the outside of the measurement region to where the stylus stocker is arranged, and the roundness measuring apparatus being configured so that when a stylus replacement command is given, the control unit carries out stylus replacement operation between the detector main unit and the stylus stocker while controlling operation of the detector driving mechanism.