A yarn break detector having a collecting electrode type sensor to which a continuously moving yarn is lightly contacted. A gate electrode of a field effect transistor is connected to the sensor and a relay circuit is connected in series to a drain electrode of the transistor. Normally, the field effect transistor of high input impedance is operated to its one condition by the collected electric charges from the continuously moving yarn. When the yarn break or cutting off occurs, the collected charges on the sensor disappear and then the field effect transistor is changed into its other condition, thereby the relay circuit produces an alarm signal. |