A device for automatic insertion of a flat elastic strip in the seam forming the edges of the corners of a fitted sheet comprises essentially and successively upstream of a sewing machine: a flat guide (1) which can be adjusted sideways and from front to back for guiding the flat fabric in the alignment of the following stations, a fabric ejector (2) the lateral outlet and inlet of which are controlled by a double-acting cylinder, a fabric detector (3), a first folder (4) which folds the edge of the fabric through 180°, a second folder (5) which folds the fabric folded through 180° through a further 180°, an elastic injector (6). Application: simplification of the sewing operations involved in the manufacture of fitted sheets.