PURPOSE: To provide the image reader in which a size of an original is detected without use of an exclusive sensor to detect a width of the original and which is realized at a low cost without simple configuration with excellent manufacture performance.

CONSTITUTION: A black mark plate 8 whose part opposite to a close contact sensor 1 has a reflectance different from that of a rear roller 9 is fixed to an original guide 7a striking on a side edge of an original 2 while the black mark plate 8 is projecting up to between the close contact sensor 1 and the rear side roller 9. Prior to reading the original 2, when the original 2 is not in existence between the close contact sensor 1 and the rear side roller 9, the close contact sensor 1 implements reading. A black mark detection section senses the position of the black mark plate 8 based on a signal outputted from the close contact sensor 1. Then an original width deciding section decides the width of the original 2 based on the position of the black mark plate 8, that is, the size of the original 2.