A method for operating a rotary die cutter comprising: providing a magnet roller; providing an anvil roller arranged in parallel with and opposite to the magnet roller with a gap therebetween; providing a single sheet-like flexible die mounted on the magnet roller; providing a pair of feed rollers (23a, 23b) spaced from the pair of magnet and anvil rollers (16, 17); providing a first drive mechanism rotating the magnet and anvil rollers (16, 17) in such a way that the magnet and anvil rollers (16, 17) are constantly rotated synchronously with each other at an equal circumferential velocity, providing a second drive mechanism rotating the pair of feed rollers (23a, 23b); providing a controller controlling the first and second drive mechanisms (18-20, 24-27), the magnet and anvil rollers (16, 17) being rotated in a direction to receive a sheet from the pair of feed rollers (23a, 23b); and providing a sensor arranged between the pair of magnet and anvil rollers (16, 17) and the pair of feed rollers (23a, 23b) to detect the passage of a leading end of the sheet, characterised in that every time a detection signal is outputted from the sensor, the pair of feed rollers (23a, 23b) is alternately rotated in the direction to convey the sheet to the pair of the magnet and anvil rollers (16, 17) and the reverse direction at a predetermined timing corresponding to a peripheral velocity and a rotational position of the flexible die so that a plurality of areas (G1-G3) spaced from each other in the conveying direction on the sheet are punched by the flexible die while the sheet is conveyed between the magnet and anvil rollers (16, 17) by the pair of feed rollers (23a, 23b), wherein the punching is performed while the sheet is alternately advanced toward and retreated from the pair of magnet and anvil rollers (16, 17), and wherein the pair of magnet and anvil rollers (16, 17) makes two or more revolutions while the sheet passes through the pair of magnet and anvil rollers (16, 17). providing a magnet roller; providing an anvil roller arranged in parallel with and opposite to the magnet roller with a gap therebetween; providing a single sheet-like flexible die mounted on the magnet roller; providing a pair of feed rollers (23a, 23b) spaced from the pair of magnet and anvil rollers (16, 17); providing a first drive mechanism rotating the magnet and anvil rollers (16, 17) in such a way that the magnet and anvil rollers (16, 17) are constantly rotated synchronously with each other at an equal circumferential velocity, providing a second drive mechanism rotating the pair of feed rollers (23a, 23b); providing a controller controlling the first and second drive mechanisms (18-20, 24-27), the magnet and anvil rollers (16, 17) being rotated in a direction to receive a sheet from the pair of feed rollers (23a, 23b); and providing a sensor arranged between the pair of magnet and anvil rollers (16, 17) and the pair of feed rollers (23a, 23b) to detect the passage of a leading end of the sheet, characterised in that every time a detection signal is outputted from the sensor, the pair of feed rollers (23a, 23b) is alternately rotated in the direction to convey the sheet to the pair of the magnet and anvil rollers (16, 17) and the reverse direction at a predetermined timing corresponding to a peripheral velocity and a rotational position of the flexible die so that a plurality of areas (G1-G3) spaced from each other in the conveying direction on the sheet are punched by the flexible die while the sheet is conveyed between the magnet and anvil rollers (16, 17) by the pair of feed rollers (23a, 23b),