A measuring method of a shape of a cross-sectional circle, which is orthogonal to an axis of a cylinder, by the following steps (i) to (vi) using measuring means which is constructed of a cylinder support jig, and a mount which has three or more sensors for detecting displacements which are located on the same cross-section orthogonal to a rotation axis of the cylinder, and outside the cylinder, are oriented to a measurement reference point (O 0) which is an intersection point of the rotation axis of the cylinder, and the cross-section orthogonal to the rotation axis, and are arranged and fixed in a shape formed by connecting said measurement reference point (O 0) and said three or more sensors being a fan shape with forming a predetermined angle (θ°) mutually with θ 0 as a center, comprising: (i) a step of measuring distances (L1 to Lm) from the measurement reference point (O 0) on each detection axis to points on a circumference of the cross-sectional circle orthogonal to the axis of the cylinder by the sensors (S1 to Sm) for detecting m pieces of displacements, wherein m is three or more; characterized in that it further comprises the following steps: (ii) a step of rotating the cylinder by the predetermined angle (θ°) in a direction of a first sensor S1 to a second sensor S2, and again measuring the distances from the measurement reference point (O 0) on each detection axis to the points on the circumference of the cross-sectional circle orthogonal to the axis of the cylinder by the sensors for detecting displacements, (iii) a step of calculating a position of a floating center O' from the change of measurements by the sensors for detecting displacements of the cylinder before and after rotation by the predetermined angle (θ°), wherein the floating center O' is a virtual center of the cross-sectional circle which may move by the rotating of the cylinder; (iv) a step of calculating a distance (O 0 - O') on an axis of the sensor Sm for detecting a displacement and measuring new distances from O' on the axis of the sensor Sm for detecting a displacement to the points on the circumference of the cross-sectional circle which is orthogonal to the axis of the cylinder, (v) a step of measuring n pieces of distances by repeating said steps (ii) to (iv), and (vi) a step of calculating a circumferential shape of the cross-sectional circle using the respective distances obtained. (i) a step of measuring distances (L1 to Lm) from the measurement reference point (O 0) on each detection axis to points on a circumference of the cross-sectional circle orthogonal to the axis of the cylinder by the sensors (S1 to Sm) for detecting m pieces of displacements, wherein m is three or more; characterized in that it further comprises the following steps: (ii) a step of rotating the cylinder by the predetermined angle (θ°) in a direction of a first sensor S1 to a second sensor S2, and again measuring the distances from the measurement reference point (O 0) on each detection axis to the points on the circumference of the cross-sectional circle orthogonal to the axis of the cylinder by the sensors for detecting displacements, (iii) a step of calculating a position of a floating center O' from the change of measurements by the sensors for detecting displacements of the cylinder before and after rotation by the predetermined angle (θ°), wherein the floating center O' is a virtual center of the cross-sectional circle which may move by the rotating of the cylinder; (iv) a step of calculating a distance (O 0 - O') on an axis of the sensor Sm for detecting a displacement and measuring new distances from O' on the axis of the sensor Sm for detecting a displacement to the points on the circumference of the cross-sectional circle which is orthogonal to the axis of the cylinder, (v) a step of measuring n pieces of distances by repeating said steps (ii) to (iv), and (vi) a step of calculating a circumferential shape of the cross-sectional circle using the respective distances obtained. (ii) a step of rotating the cylinder by the predetermined angle (θ°) in a direction of a first sensor S1 to a second sensor S2, and again measuring the distances from the measurement reference point (O 0) on each detection axis to the points on the circumference of the cross-sectional circle orthogonal to the axis of the cylinder by the sensors for detecting displacements, (iii) a step of calculating a position of a floating center O' from the change of measurements by the sensors for detecting displacements of the cylinder before and after rotation by the predetermined angle (θ°), wherein the floating center O' is a virtual center of the cross-sectional circle which may move by the rotating of the cylinder; (iv) a step of calculating a distance (O 0 - O') on an axis of the sensor Sm for detecting a displacement and measuring new distances from O' on the axis of the sensor Sm for detecting a displacement to the points on the circumference of the cross-sectional circle which is orthogonal to the axis of the cylinder, (v) a step of measuring n pieces of distances by repeating said steps (ii) to (iv), and (vi) a step of calculating a circumferential shape of the cross-sectional circle using the respective distances obtained.