PURPOSE: To provide a yarn detecting device for doubler capable of eliminating the cause of a yarn failure by detecting the state of a yarn to be doubled, knowing the cause of the yarn failure according to the state of the yarn, and operating a control device performing an operation coping with the cause.

CONSTITUTION: In a doubler for doubling yarns 3, 6 supplied from two packages 2, 5, fillers 13, 14 for detecting the traveling states of the respective yarns 3, 6 supplied from the packages 3, 5 are disposed on the traveling passages of the yarns 3, 6, respectively, the state of the yarns at the time of a yarn failure are detected by the respective fillers 13, 14, and a control unit 20 for controlling operations such as supplied yarn exchange or cutter operation signal.