Abstract: A sensor (10) for measurement of the deflection of a machine element comprises a detecting portion, a light source, a light detector, and connected guides. An electronic data processing unit is connected to the sensor. The detecting portion comprises a tuft of optical fibers, a focusing lens fitted into a tube, and a mirror separated from the tube. The deflection of the machine element (17) caused by the work piece results in an inclination of the mirror relative to the center line of the tube, and therefore a light beam from the light source is reflected in the mirror towards a point on the end of the fiber tuft which is arranged at a distance from the center line. This reflected light is transferred to the light detector for transmission of an analogous signal to the electronic unit in which the value of the deflection is determined.