A speed detector comprises a rotary shaft (1) for rotation' with a sewing machine, a ring-shaped permanent magnet (9) uniformly magnetized in a direction parallel to the axis of the rotary shaft, at least one ring-shaped, internally toothed ferromagnetic member or stator (5) and an externally toothed ferromagnetic circular member or rotor (6) which is secured to the rotary shaft inside the stator to form a varying air gap between the opposing teeth. The permanent magnet and the two ferromagnetic members are disposed to form a closed loop magnetic circuit in which a ring-shaped coil (10) is disposed. The air gap and hence the reluctance of the magnetic circuit changes periodically with the revolution of the rotary shaft so that a voltage is generated in the coil which varies at a frequency proportional to the operational speed of the sewing machine.