A lock stitch sewing machine with an automatic thread tension adjusting device is disclosed, wherein the sewing machine has a plurality of thread tension adjusting mechanisms 7, each of which including a pair of thread tension disks 12, 13, one of which is normally pressed by a spring toward the other. A sensor 4 is responsive to the operation of a thread tension releasing lever 20 made by a user with a slight touch applied thereto to give a signal. A CPU1 is responsive to the signal to compare the value of the existing thread tension of the thread tension adjusting mechanisms 7 with a predetermined one. If the value of the existing thread tension is higher than the predetermined one, the CPU1 is operated reduce the value of the existing thread tension to a predetermined one. Thus the tension releasing operation ability is increased.