the second guide 40 is provided with a pre tension spring plate 56 and a spring arm 58 to apply a predetermined sliding resistance to the needle thread 32 and to prevent the needle thread 32 from being broken when the tension on the needle thread is increased temporarily the maximum tension that is exerted on the needle thread 32 is limited for example to a tension between the breaking tension of a 30 1 cotton thread and a tension required for forming satisfactory stitches in sewing denim of 5 mm in thickness thus spring arm 58 functions as a buffer as illustrated in fig2 the arm shaft 22 is supported rotatably at one end in a bearing bush 60 on the frame 14 and a cam member 62 is fixedly mounted on the arm shaft 22 near the end of the same a first annular groove 64 and a second annular groove 66 are formed in the circumference of the cam member 62 the bottom surface of the first annular groove functions as a first cam for controlling the needle thread clamping device 36 the second annular groove 66 is a groove cam hainv a side wall functioning as a second cam for controlling the motion of the take up lever 42 the first and second cams are designed so as to control the needle thread clamping device 36 and the take up lever 42 respectively for motions indicated by motion curves in fig3 a needle thread clamp control plate 68 is mounted on the arm shaft 22 for rotation relative to and coaxially with the arm shaft 22 between the cam member 62 and.