A yarn monitoring device (34) includes a yarn irregularity sensor (37) and a yarn monitoring controller (81). The yarn irregularity sensor (37) monitors a yarn (21) that is being wound by an automatic winder (1). The yarn monitoring controller (81) performs a determination based on information acquired from the yarn irregularity sensor (37) and information pertaining to, for example, a traverse position of the yarn (21) acquired from the yarn detection sensor (71) of the automatic winder (1), and outputs a cutter operation signal based on the determination to operate a cutter (39). Furthermore, the yarn monitoring device (34) includes a yarn detection receiving section (83) to receive a yarn detection signal output by the yarn detection sensor (71).