PURPOSE: To accurately measure fluctuations in the diameter of a moving strand by allowing laser beam to impinge against the moving strand to generate diffraction and interference patterns to detect them by a DCD and calculating the diameter of the strand on the basis of the output of the CCD.

CONSTITUTION: Two or more pairs of lasers 4 and CCDs 5 are arranged in the detection chamber 8 of a measuring apparatus 1 at known angular positions. A moving strand 9, for example, a moving glass fiber is inserted into the detection chamber 8 to be aligned with the center line of the chamber. Subsequently, radiations are allowed to impinge against the strand 9 from the respective lasers 4 to generate diffraction and interference patterns which are, in turn, detected by the mutually opposed CCDs 5. The detection signals are subjected to analogue/digital conversion to be transmitted to a memory control panel and predetermined calculation is performed by a computer to calculate the diameter of the strand. Therefore, the diameter of the moving strand can be accurately measured and this method is usefully used in fiber industry relation.