To provide a yarn sensor, eliminating influence of variation in intrinsic detection characteristics by correction, and requiring no setting of the correction information at every replacement.

This yarn sensor 24 includes: a detecting part 40 for transmitting a detection signal corresponding to the running speed of a running yarn Y; a storage part 41 for storing the correction information inherent to the detecting part 40; an operation part 42 for correcting the detection signal transmitted from the detecting part 40 using the correction information stored in the storage part 41; and a signal output part 43 for outputting the already corrected signal corrected by the operation part 42.