A locking mechanism includes a hook, an engaged portion, and a biasing member, and locks a movable unit supported on a unit main body to the unit main body. The hook has a supporting portion pivotably supported on the unit main body and an engaging portion protruding from the supporting portion. The engaged portion is formed on the movable unit and is engageable with the engaging portion. A biasing member biases the hook in such a direction as to let the engaging and engaged portions engage. The hook has a first face facing the engaged portion at the tip end of the engaging portion and a second face extending from the first face toward the supporting portion and inclined relative to the first face in such a direction as to recede from the pivot. With the engaging and engaged portion in engagement, the engaged portion touches only the second face.