To provide a corner sewing method and a flat sewing machine for corner sewing capable of preventing skipping of stitches generated at the time of sewing again after a fabric is turned around so as to obtain finished sewing items having good appearance.

The flat sewing machine comprises a plurality of needles for inserting each needle thread separately and moving vertically; a needle plate having a claw formed so as to cross between a plurality of needle location parts; a looper for inserting looper thread, and elliptically moving between a forward position and a backward position along a lower face of the needle plate; a releasing mechanism for releasing supporting of thread loops supported by the claw; and a rotation detecting device for detecting rotating states of a sewing machine main shaft. At the time of sewing by the flat sewing machine, the looper thread extending from the looper in the forward position to the fabric is caught by a hook based on signals generated at the time of stopping of sewing and detected by the rotation detecting device. Then the releasing mechanism is operated to release supporting of the thread loops by the claw. After the fabric is turned around and sewing is started again, catching of the looper thread by the hook is released and the thread loops are supported by the claw.