Seam ravel preventing method, seam ravel preventing apparatus and seam structure

A seam ravel preventing apparatus for preventing ravel of a seam of multi-thread chain stitching, said seam ravel preventing apparatus being installed in a sewing machine which comprises a needle that moves up and down while holding a needle thread and a looper that holds a looper thread and advances and retracts in a direction substantially orthogonal to an up-down movement pathway of the needle and in which a needle thread loop formed under a needle plate by the needle is caught by the advancing looper and the needle thread loop is subjected to inter-looping with the looper thread held by the looper to form a seam of multi-thread chain stitching on a cloth, said seam ravel preventing apparatus comprising:

• a needle-thread holding mechanism that approaches to and recedes from the looper, and holds a needle thread loop caught by the looper on an advance end side of the looper away from a fall position of the needle when the needle-thread holding mechanism approaches to the looper;
• a looper-thread holding mechanism that approaches to and recedes from the looper, and holds a looper thread extending from the looper to a cloth on a front side away from the fall position of the needle;
• and
• a control section for controlling approaching/receding of the needle-thread holding mechanism and the looper-thread holding mechanism in association with operations of the needle and the looper and feeding of the cloth, wherein
• after first sewing is completed with the looper located at an advance position thereof and the needle located at a rise position thereof, the control section causes the needle-thread holding mechanism and the looper-thread holding mechanism to approach to the looper and hold a needle thread loop and a looper thread respectively, and provides control of maintaining a state where the needle-thread holding mechanism and the looper-thread holding mechanism hold the needle thread loop and the looper thread respectively until the needle falls and passes through the needle thread loop and performing second sewing for at least one stitch in the maintained state while synchronizing falling/rising of the needle with advancing/retracting of the looper and the feeding of the cloth.