A contact image sensor is received and held in a holder slidably mounted on a rectangular lower frame and is disposed in parallel to the two short sides of the lower frame to pick up the image of a detection object placed on the transparent plate mounted on a rectangular upper frame. A rod-like guiding member having two ends respectively mounted on the center of the two short sides of the lower frame, disposed orthogonal to the longitudinal holder, for guiding the movement of the holder is provided. A connecting member slidably mounted on the guiding member and detachably connected to the holder near the center thereof is provided. A biasing member for biasing the holder upwardly, disposed between the connecting member and the holder is also provided.