To enable measurement of an outer diameter and a correct roundness by a traveling apparatus, by measuring the outer diameter at a plurality of points on the same circumference of a tubular body, columnar body, etc., while the apparatus is made running at the same speed in the same direction as the tubular body, columnar body, etc.

When a measurement start signal is generated, a pipe 1 is chucked and the whole of a ravel cart 5 is synchronized with a travel speed of the pipe 1. The outer diameter is read from size-measuring devices 2a, 2b always measuring the outer diameter and stored in a memory device. A driving device 7 is started and stopped by one cycle to rotate a rotary frame 3 by a preliminarily set angle. Whether a prelminarily set count of points is completely measured or not is judged. When the measurement is completed, a chuck 9 is freed and an air cylinder is retreated, so that the whole travel cart 5 is returned to an original position. In consequence, the outer diameter can be measured at a plurality of points on the same circumference. A true roundness can be correctly and stably measured while the tubular body, columnar body, etc., is moving.