Provided is a structure capable of preventing shaking of an image reading unit (1) during movement at low cost. A first contact portion (T1) between the image reading unit (1) and a guide shaft (2A) supports force (F1) acting in a gravity direction and force (F2) acting in a horizontal direction in the case where the image reading unit (1) moves in a predetermined direction. In this case, the first contact portion is arranged so as to satisfy F1>F2×tan θ1, where θ1 represents an angle formed by a perpendicular line (γ) with respect to a tangential plane (β) of the guide shaft (2A) and the horizontal direction. Thus, the image reading unit (1) can be prevented from being lifted due to rotation moment during movement.