A thread cutting unit cooperating with the bed of a sewing machine includes a lower shaft, a cutting knife and a loop spreading plate respectively fixed to the lower shaft and positioned adjacent to the hook. The thread cutting knife and the loop spreading plate cross each other in a space between the hook and the throat plate. The thread cutting unit is provided with a thread cutting driving unit which comprises a thread cutting knife driving shaft, a loop spreading plate driving shaft, a rotational motion transmission device, a thread cutting supporting member carrying thread cutting knife, a first turnable interlocking mechanism and a second turnable interlocking mechanism. The thread cutting supporting member is rotatably supported on the bed.