An inner circumferential length measurement method for a circular member , comprising the steps of: disposing a two-dimensional sensor at a predetermined measurement position on an inner side of the circular member placed flat in an unrestrained state on a support , such that the two-dimensional sensor faces an inner circumferential surface of the circular member in a non-contact state; rotating the two-dimensional sensor about a predetermined position on the inner side of the circular member ; measuring a separation distance from the two-dimensional sensor to the inner circumferential surface around an entire circumference of the circular member ; and calculating an inner circumferential length of the circular member via a calculation unit using the separation distance measured and a distance from the predetermined position to the two-dimensional sensor in a plan view, characterized in that the support is raisable from a horizontal state to an upright state and a surface of the support is provided with projection portions spaced from one another, the projection portions projecting from the surface; the circular member is placed flat in an unrestrained state on the support by, in a case where the support is in an upright state, engaging the inner circumferential surface of the circular member with two of the projection portions to place the circular member on the support and putting the support in a horizontal state; the projection portions are retractable from the surface of the support ; and the projection portions are retracted below the surface of the support after the circular member is placed flat on the support . disposing a two-dimensional sensor at a predetermined measurement position on an inner side of the circular member placed flat in an unrestrained state on a support , such that the two-dimensional sensor faces an inner circumferential surface of the circular member in a non-contact state; rotating the two-dimensional sensor about a predetermined position on the inner side of the circular member ; measuring a separation distance from the two-dimensional sensor to the inner circumferential surface around an entire circumference of the circular member ; and calculating an inner circumferential length of the circular member via a calculation unit using the separation distance measured and a distance from the predetermined position to the two-dimensional sensor in a plan view, characterized in that the support is raisable from a horizontal state to an upright state and a surface of the support is provided with projection portions spaced from one another, the projection portions projecting from the surface; the circular member is placed flat in an unrestrained state on the support by, in a case where the support is in an upright state, engaging the inner circumferential surface of the circular member with two of the projection portions to place the circular member on the support and putting the support in a horizontal state; the projection portions are retractable from the surface of the support ; and the projection portions are retracted below the surface of the support after the circular member is placed flat on the support . <EOS>