An auto-lock mechanism to be mounted on an image input/output device includes a first swingable element and a second swingable element. When the image input/output device is not slantwise and an upper cover of the device is opened, one of the first and second swingable elements swings but does not hook the other of the first and second elements. When the image input/output device is slantwise, both of the first and second swingable elements simultaneously swing, approach and hook each other such that the upper cover remains closed and is free from detachment to hurt a user.