To provide a reliable paper width detecting device capable of accurately detecting paper width and preventing a human error such as size setting mistake by an operator with the necessary minimum number of detecting means.

A paper feeder provided in an image forming device to feed a rolled paper 2 has a cutter 4 for cutting the rolled paper 2 at a desirable length. The cutter 4 is structured of a fixed blade 17, a moving blade 18 for abutting on the fixed blade 17 to cut the rolled paper, and a slider 28 for sliding in the paper sheet cross direction, while holding the moving blade. The paper width detecting device has a reflection type paper sheet detecting means 29 and a paper sheet detecting time counting means 39 for counting the paper sheet detecting time when the slider moves near the slider 28, and computes the paper width on the basis of the time counted by the paper sheet detecting time counting means 39.