An image forming device includes a case whereon a medium gateway opening is formed, and a tray assembly. The tray assembly includes a tray, a cover and a resilient component. The tray drives the cover to pivot relative to the case in a first pivoting direction from a second folding position to a second using position to reveal the medium gateway opening when the tray pivots relative to the case in the first pivoting direction from a first folding position to a first using position. When the tray pivots relative to the case in a second pivoting direction opposite to the first pivoting direction from the first using position to the first folding position, the cover is driven by the resilient component to pivot relative to the case in the second pivoting direction from the second using position to the second folding position to conceal the medium gateway opening.