To automatically and accurately set each size of naps to be detected depending on species of threads in a nap-detecting device.

In a nap-detecting device 1 which judges the presence of naps by generating a nap detection signal by detecting the nap of a thread 8 traveling in a fiber machine 21 by a nap detector 2 and comparing the level of the detected nap detection signal with that of predetermined level of the nap judgment signal by comparators 10, 11, a nap judgment level-setting device 5, which sets the level of the nap detection signal in the comparators 10, 11, consists of a memory means 18 which memorizes the nap judgment signal level dependent on the species of threads, an input means 16 which inputs the data corresponding to thread species and a selection means 17 which selects a nap judgment signal level from the memory means 18 corresponding to the data from the input means 16, outputs the signal to the comparators 10, 11 and automatically sets the nap judgment signal level corresponding to the thread species.