An optical fiber, being drawn from a preform and wound upon a reel, passes between parallel plates of a capacitor connected in a measuring circuit which, from the capacitance variations due to changes in fiber diameter, controls a reel-driving motor to minimize these variations. The capacitor may have a constant charge, as by having one of its plates made of an electret, in which case the capacitance variations are determined from voltage changes. A calculator connected to the measuring circuit translates these variations into numerical values representing the fiber diameter which can be visually displayed.