Abstract: A needle thread restraining device in a thread cutting sewing machine comprising a needle thread restraining device 18 which is disposed on a needle thread path between a needle take-up mechanism (5) and a needle thread supplying source (6) for setting the tension of the needle thread 4 to substantially zero when the thread cutting mechanism (9) does not operate and to a given value when the thread cutting mechanism (9) operates, and a tension nullification means 31 for setting the tension applied by a tension regulator (10) to a given value when the thread cutting mechanism (9) does not operate and for nullifying the tension applied by the tension regulator when the thread cutting mechanism operates. The amount of the needle thread to be drawn under the work can be set properly and independently, nullifying of the tension set by the tension discs when the sewing thread is cut by the thread cutting mechanism.