An image reading apparatus includes a line light source having a light guide plate and red-, green-, and blue-LEDs, for a light transmitting original, and a contact image sensor unit for detecting light from the line light source. The light transmitting original is arranged between the line light source and the contact image sensor unit. The light transmitting original is read by moving the line light source and the contact image sensor unit relative to the light transmitting original. The line light source is moved interlockingly with the contact image sensor unit by the attraction between a magnet provided at both ends of the line light source in the longitudinal direction thereof and a magnet provided at both ends of the contact image sensor unit in the longitudinal direction thereof.