A form measuring apparatus comprising: a probe configured to measure a measured object; a rotary table on which the measured object is to be placed and which is rotatable relative to the probe; and a coordinate system calculator configured to: calculate coordinate axes to establish a coordinate system for the rotary table; calculate, based on a position of at least one master ball (61; 62) fixed to the rotary table, a center of a circle (C1; C2) traced by the master ball (61; 62) when the rotary table is rotated; and calculate a coordinate system of the rotary table having the center of the circle (C1; C2) as an origin point (O T); characterized in that the coordinate system calculator is configured to correct coordinates Xc, Yc, Zc of the origin point (O T) based on a calibrated inner diameter value Dc of a ring gauge fixed to the rotary table, a first inner diameter value D1 of the ring gauge measured by a first measurement in which the probe approaches the ring gauge in a first direction, and a second inner diameter value D2 of the ring gauge measured by a second measurement in which the probe approaches the ring gauge in a second direction, wherein the coordinate system calculator calculates, based on the inner diameter values Dc, D1 and D2, Xt1 and Yt1 represented by formulas below: Xt 1 = − Dc + 2 M 1 + 2 D 1 2 − Dc 2 + 4 DcM 1 − 4 M 1 2 4 and Yt 1 = Xt 1 + M 1 wherein M1 is represented by a formula below: M 1 = D 2 2 − D 1 2 / 4 Dc and the coordinate system calculator calculates true coordinate values Xc1, Yc1 by correcting the coordinate values Xc, Yc in the coordinates Xc, Yc, Zc of the origin point (O T), using Xt1 and Yt1, as shown in formulas below: Xc 1 = Xc − Xt 1; and Yc 1 = Yc − Yt 1, wherein: the probe is drivable so as to run parallel to each of an Xs axis, a Ys axis, and a Zs axis, which are mutually orthogonal, a rotation axis of the rotary table is parallel to the Zs axis, the first direction is parallel to the Xs axis, as viewed in the Zs axis direction, and the second direction is parallel to the Ys axis, as viewed in the Zs axis direction, the form measuring apparatus further comprises a coordinate system register configured to register the coordinate system of the rotary table. a probe configured to measure a measured object; a rotary table on which the measured object is to be placed and which is rotatable relative to the probe; and a coordinate system calculator configured to: calculate coordinate axes to establish a coordinate system for the rotary table; calculate, based on a position of at least one master ball (61; 62) fixed to the rotary table, a center of a circle (C1; C2) traced by the master ball (61; 62) when the rotary table is rotated; and calculate a coordinate system of the rotary table having the center of the circle (C1; C2) as an origin point (O T); calculate coordinate axes to establish a coordinate system for the rotary table; calculate, based on a position of at least one master ball (61; 62) fixed to the rotary table, a center of a circle (C1; C2) traced by the master ball (61; 62) when the rotary table is rotated; and calculate a coordinate system of the rotary table having the center of the circle (C1; C2) as an origin point (O T); characterized in that the coordinate system calculator is configured to correct coordinates Xc, Yc, Zc of the origin point (O T) based on a calibrated inner diameter value Dc of a ring gauge fixed to the rotary table, a first inner diameter value D1 of the ring gauge measured by a first measurement in which the probe approaches the ring gauge in a first direction, and a second inner diameter value D2 of the ring gauge measured by a second measurement in which the probe approaches the ring gauge in a second direction, wherein the coordinate system calculator calculates, based on the inner diameter values Dc, D1 and D2, Xt1 and Yt1 represented by formulas below: Xt 1 = − Dc + 2 M 1 + 2 D 1 2 − Dc 2 + 4 DcM 1 − 4 M 1 2 4 and Yt 1 = Xt 1 + M 1 wherein M1 is represented by a formula below: M 1 = D 2 2 − D 1 2 / 4 Dc and the coordinate system calculator calculates true coordinate values Xc1, Yc1 by correcting the coordinate values Xc, Yc in the coordinates Xc, Yc, Zc of the origin point (O T), using Xt1 and Yt1, as shown in formulas below: Xc 1 = Xc − Xt 1; and Yc 1 = Yc − Yt 1, the probe is drivable so as to run parallel to each of an Xs axis, a Ys axis, and a Zs axis, which are mutually orthogonal, a rotation axis of the rotary table is parallel to the Zs axis, the first direction is parallel to the Xs axis, as viewed in the Zs axis direction, and the second direction is parallel to the Ys axis, as viewed in the Zs axis direction, the form measuring apparatus further comprises a coordinate system register configured to register the coordinate system of the rotary table.