PURPOSE: To monitor end breakage of single yarn in the case of end down of constituent single yarn in a yarn doubling process for doubling plural single yarns.

CONSTITUTION: Yarn unevenness of the whole multifilament during yarn doubling is detected in every given length L of the yarn and the number N of yarn unevenness exceeding a given sensitivity level ±α% in the given length L is calculated. The average number Nm of yarn unevenness calculated during normal traveling of multifilament yarn is obtained and is compared with the number N of yarn unevenness calculated during the monitoring. When N<Nm, the yarn doubling is judged as end breakage of single yarn.