A pocket opening sewing machine including a cutting device for producing two notching cuts at the ends of a pocket opening. The sewing machine has sewing needles and a belt driven clamping device for clamping to the sewn parts for advancing them in an advancing direction. The cutting device includes two knives spaced apart. The knife that is further from the sewing needles is movable toward the stationary knife that is closer to the needles. For adjusting the spacing between the knives, the movable knife is temporarily clamped to a moving belt which moves it toward the stationary knife. An adjustably positionable stop defines the furthest limit of spacing between the stationary and movable knives and a spring drives the movable knife against the stop. The clamping of the movable knife to the belt is initiated by a sensor which senses the passage of a sewn part and which operates a microcomputer with pocket length information stored in it to move the movable knife when required over the distance required. Alternately, if the movable knife moves at a speed faster than the advancing speed of the sewn parts, adjustment of the movable knife stop can be dispensed with.