A roundness measurement device 1 according to the present invention includes a base 10, a stage 12 provided on the base 10 and configured to place a workpiece W thereon, a column 14 elected on the base 10, a carriage 16 supported movably along the column 14, a turning arm 18 supported by the carriage 16 turnably around a pivot axis in a radial direction (left-right direction), a radial direction moving arm 20 supported movably in the radial direction, and a detector holder 22 fixed to a tip end of the radial direction moving arm 20. A detector 24 is held by the detector holder 22, and a position of the detector in the radial direction is adjusted by movement of the radial direction moving arm 20 in the radial direction.