Device for detecting tension and use of seat belt

The seat belt has an associated closure element (20) whose movement into the locking opening (3) is monitored through a magnetic circuit (16) containing a sensor (31) which is sensitive to a magnetic field. A measuring element (25) associated with a part (11) of the belt lock (8) fixing the fixing element (2) is used to detect via the magnetic circuit (16) the force acting on the belt lock (8). The closure element which passes through the locking opening of the fixing element is preferably made of Ferro-magnetic material.