Provided is a faulty sewing detector that is, for example, mounted on an overlock sewing machine, with which an over-edge chain stitch is made in a state in which a body fabric and a sleeve fabric of a T shirt or underwear are overlapped with each other. The detector comprises a fabric twisting detecting sensor that detects a fabric twisting by determining the number of overlap of fabrics sewed which is disposed at a position corresponding to the fabric sewing area of a needle, and, behind a needle location in a sewing direction; a faulty sewing judging means that compares a fabric twisting quantity detected by the sensor, with a preset permissible fabric twisting quantity, to judge it as faulty sewing when the former exceeds the latter; and an informing means that is activated when judged as faulty sewing. With this construction, a faulty sewing due to slip out even in performing a curve stitch can be detected reliably, and a variety of attachments, as required, can be mounted freely in front of the needle location.
