To provide a thread detector capable of securely detecting the positional change in a crosswise direction of threads as well as the running state of the threads.

A thread detector 1 comprises a light source 17 for irradiating a thread and a plurality of thread detecting parts 18 disposed with a prescribed interval in a running direction of thread capable of changing an output signal in response to intensity of a received light from the light source 17. The thread detecting part 18 is made into a belt-like shape extending in a crosswise direction orthogonal to the running direction. The light source 17 is installed so as to differentiate the amount of light received in the crosswise direction. The thread detector 1 further comprises a calculation unit 19 for calculating the running state and the positional change in the crosswise direction of thread on the basis of the output signal in response to the amount of light received at the thread detecting part 18.