PURPOSE: To determine indicators for various characeristics of a filament such as u% and deviation coefficient of slugs by processing a thickness signal with a microcomputer after it is memorized in a time sharing manner with a sampling in a slugs analysis of filament.

CONSTITUTION: The thickness of a running filament 1 is detected with a capacitor 2 and appied to an amplifier 3. An analog-digital converter 4 converts the thickness signals into digital signals at an interval of a fixed time according to the running speed of the filament and the length (d) of a electrode under the control of a timer 5. Thickness information which is stored into a memory 8 in a time sharing manner one after another is processed with a CPU6 controlled by a program in a program memory 7. A mean X is obtained from thickness data Xi (i=1Wh) to compute u%=ΣXi-X/X...100 for indicating the slugs by deviation from the mean value. This also enables the calculation of deviation coefficient by an equation CV={Σ(Xi-X)2/n}1/2/X, the proportion of parts of the filament exceeding the mean 1.5 times, and the like.