Abstract: The apparatus for controlling the warp thread tension by positional displacement of a back rest on a loom includes a swivelling lever mechanism with a rocking lever which carries the shaft of the back rest and which rocking lever is controlled by a cam plate or disc. The apparatus also includes additional members effecting an additional warp thread tensioning by an additional positional displacement of the back rest during the start-up operation of the loom. The swivelling lever mechanism further includes a connecting element which serves for altering the relative distance between the cam plate or disc and the rocking lever carrying the shaft of the back rest. The relative length of the connecting element is variable by a temporarily operative actuating device. These measures permit a very simple and functionally reliable concept of the additional members of the apparatus.