To obtain a sensor for measuring tension of a thread accurately even when the thread is traveling at high speed.

The thread tension sensor comprises a movable thread guide 12 projecting from the movable part 32 of a metal plate 3 secured to a case 2 at the opposite ends thereof, a magnet 4 fixed to a shaft 14 disposed on the lower surface of the movable part 32, and a reluctance element 5 disposed oppositely to the magnet 4 at a fixed position. When a thread 8 entraining about the movable thread guide 12 travels at high speed, the movable thread guide 12, the movable part 32 and the magnet 4 are moved integrally to a position where the tension is balanced with the resiliency of the metal plate 3. Output voltage of the reluctance element 5 is varied based on the movement of the magnet 4.