An image reading device which is capable of reading out image data on an original while conveying the original in a simple construction. A conveying path has a pair of guide members facing each other at a distance and extending in a conveying direction of an original. An original conveying unit conveys the original along the conveyance path. An image reading unit is comprised of a flexible sheet-like substrate on which a plurality of optical sensors are arranged. The sheet-like substrate is inclined with respect to the conveying direction of the original, and fixed to one of the pair of guide members at one end thereof and abuts against the other of the pair of guide members at the other end thereof. The image reading unit reads image data on one side of the original when the original is conveyed along the conveying path and passes through between the other end of the sheet-like substrate and the other of the pair of guide members.