an image reading apparatus including a transparent original table for placing a transmitting original thereon , an image reading unit for reading the image of the transmitting original through the transparent original table , a transmitting original holder having a through - hole for positioning the transmitting original relative to the transparent original table , and an illuminating unit adapted for pressing the transmitting original positioned by the through - hole of the original holder , including a plane light source for illuminating the transmitting original , and comprising a fitting portion for fitting in the through - hole , and a protruding portion protruding toward the surface of the transmitting original from the through - hole on a side far from the transmitting original relative to the fitting portion , wherein when the transmitting original illuminating unit is inserted into the through - hole of the transmitting original holder at an angle at which the surface having the plane light source is not parallel to the transparent original table , the protruding portion of the transmitting original illuminating unit first contacts with the edge of the through - hole of the transmitting original holder .