Abstract: An apparatus according to the present invention comprises a sensor and a control circuit. The sensor is disposed near a needle drop portion of a needle plate and detects the quantity of light transmitted through cloths passing over the needle plate. The control circuit determines defective sewing from increase and decrease of said light quantity. The control circuit comprises means for storing the averaged value of the light transmitted in a few stitches of initial sewing operation, and means for determining defective sewing continuously from the initial sewing operation by comparing the average value with the value of the light transmitted and detected each time. Thus, it is possible to immediately start the practical sewing operation without taking time for test sewing or other extra jobs even when every cloth to be sewn is different in properties.

Abstract: In a needle plate used in a sewing machine for forming double chain stitches, a recess is formed at a looper heel side of a needle hole behind a needle drop point. During the sewing of a fabric, the loop of the looper thread expands by tensioning of the needle thread and settles within the recess, so that balloon stitches are formed on the under side of the fabric.