To improve sewing quality and operability by shortening an idle sewing distance without generating malfunctions in any stitch interval.

In the case of judging that bobbin thread is broken (Yes in S6) based on contact pressure generated when the bobbin thread is brought into contact with a bobbin thread tension sensor during the sewing of a sewing machine, a sewing distance Lc at the time is summed up and the drive of the sewing machine is stopped (S10) at the time of judging that the summed-up sewing distance becomes more than a thread breakage judgement distance Lb determined beforehand (Yes in S9.) On the other hand, in the case of judging that the bobbin thread is not broken (No in S6,) the sewing distance Lc summed up until then is cleared and judgement is prevented from being made as thread breakage in S9.