PURPOSE: To achieve a higher accuracy in the detection of the thickness of a cable cover by measuring the thickness of the covering of a reference sample and a cable separately with an ultrasonic sensor after the reference sample made of material the same as the cable covering is arranged underwater.

CONSTITUTION: Plastics is extruded over a cable core 2 from an extruder 1 to make covering. In the normal process, a pulse is emitted to the cable 3 and the reference sample 6 from ultrasonic sensors 7 and 8 and reflected pulses are received with the sensors 7 and 8 to measure the thickness of the cover of the reference sample 6 and the cable 3 separately. The measured values of the covering thickness undergo a specified computation with a computing unit 9 to determine the coving thickness of the cable 3 and the results are shown on a display 10.