A straightness measuring instrument includes a base, two loading units, a center clamping apparatus, and a measure apparatus. The base includes two support rods and a sliding rod. The loading units are mounted on the base and are arranged along an axial direction of the support rods at intervals. The center clamping apparatus is mounted in the base and includes a fixed seat and a movable seat. The measure apparatus is mounted movably around the sliding rod. The loading units are adjustable for easy and quick movement depending on the length of a workpiece. Also, the center clamping apparatus can assist with clamping the workpiece that has different diameters.