Abstract: An apparatus for measuring tension in a longitudinally extending belt has a longitudinally extending housing formed with a transversely extending guide passage, a pair of longitudinally spaced abutments fixed on the housing and longitudinally flanking the passage, and a piston displaceable in the guide passage and having an end forming a support movable crosswise across a longitudinally extending line defined by the fixed supports. An abutment is displaceable crosswise in the passage between a pair of end positions. A spring in the passage is braced between the abutment and the piston. An actuating member mounted on the housing is connected to the abutment for displacing same crosswise between its end positions and thereby compressing the spring against the piston.