Thanks to the multiple knife-dropping functions, it is no longer required that the knife is changed at the buttonhole size is changed. The knife supports sewing lengths of 41mm at the maximum. In addition, knife mechanisms for 70mm and 120mm are optionally available. With this, slots such as the belt holes of child seat belts can be buttonholed. Needle thread tension for sewing parallel and bartacking sections of buttonholes can be separately controlled through the operation panel and stored in memory according to various sewing conditions (e.g., type of thread, type of material and sewing speed).