To make detectable an abnormality of outside diameter on-like over the full length of an object to be tested by operating a non-contact range finder according to the transport status of the object.

In this test method, a tubular object to be tested 1 rectilinearly moving while rotating is an object for test, and a non-contact range finder 2 is installed in the circumferential direction of the test object 1. To continuously measure the range to the outer surface of the test object over the full length of the tubular object 1 to be tested, the non-contact range finder 2 is fixedly installed. That is, the measuring point (indicated by a dotted line) is extended to the full length and whole surface of the tubular object 1 by the transport operation in the spiral direction which is a combination of rotation and rectilinear movement of the tubular object 1. The distance to the outer surface of the object 1 is continuously measured by the non-contact range finder 2, and according to the measured displacement, abnormality of outside diameter of the object 1 is detected.