To provide a method for converting the image data of a paper piece into a format suitable for identification.

This bill identifying device 2 for converting the image data of a rectangular bill moved on a conveyance path is provided with a line sensor 4 receiving the image data of the bill on a coordinate plane made of an x-axis perpendicular to the conveying direction and a y-axis along the conveying direction, an identifying CPU 8, a corner angle converting function 10 converting the image data of the bill so that corners become rectangular if the corners of the received image data of the bill are not rectangular, a rotation correcting function 12 rotatively correcting the image data of the bill to eliminated the inclination of the image data of the bill against coordinate axes, a length converting function 14 converting the length in the y-axis direction into specified value on the image data of the bill after a rotation correction, and a position moving function 16 parallelly moving the prescribed image data to a prescribed reference position.