Patent ID: 8102574

Claim:
An image reading device comprising: an exposing unit that exposes an original document to light; a photoelectric converting unit that converts an intensity of light reflected from the original document exposed to light by the exposing unit into an electrical signal, and outputs an analog image signal in a main scanning line period; a clamp unit that inputs an alternating-current-coupled analog image signal outputted by the photoelectric converting unit, and matches an offset potential of the analog image signal with a predetermined clamp potential; a processing unit that performs analog processing of the analog image signal that has passed through the clamp unit; a signal converting unit that converts the analog image signal subjected to analog processing by the processing unit into a digital image signal; a switching unit that switches the main scanning line period, and outputs analog image signals to the photoelectric converting unit at a plurality of reading speeds; and a changing unit that changes a clamp period of the clamp unit depending on the main scanning line period after switching by the switching unit.