An auxiliary film (4) is applied or fused to a slide fastener chain (1) having a pair of intermeshing rows of coupling elements (2) with an element-free space (3) defined therein. The slide fastener chain (1) is fed by a chain feeder (A) along a feed path (6). The device for applying the auxiliary film (4) includes an ultrasonic horn (8) disposed above the feed path (6), a holder (9) disposed below the feed path (6) and movable toward and away from the ultrasonic horn (8), a stopper (13) disposed in the holder (9) and movable into the element-free space (3) of the slide fastener chain (1), an anvil (14) disposed in the holder (9) and movable toward and away from the ultrasonic horn (8), and a feed roller assembly (31, 36) for supplying an elongate auxiliary film (27) onto the anvil (14). A sensor roller (48) is mounted on the holder (9) for pressed engagement with the elongate auxiliary film (27), the sensor roller (48) having a plurality of circumferentially equally spaced controlling elements (51). A controlling element sensor (52) is disposed near the sensor roller (48) for sensing the controlling elements (51) one at a time and issuing a command signal to keep the chain feeder (A) energized insofar as the controlling element sensor (52) successively senses the controlling elements (51).