A method for measuring a deflection amount of a columnar structure includes the following steps. The method includes placing a longitudinal direction of a columnar structure in a direction parallel to a ground to place the columnar structure on a support member that supports the columnar structure such that the columnar structure is rotatable around an axis in the longitudinal direction; determining that the columnar structure placed has stopped rotating under its own weight around the axis; and measuring, after the stop of the rotation is determined, measuring a distance in a vertical direction between a straight line connecting both ends in the longitudinal direction of the columnar structure and a curved line along a shape in the longitudinal direction of the columnar structure.