A steam oven including an outer casing to define an appearance of the steam oven, an inner casing provided in the outer casing to define a cooking cavity to cook food therein. The inner casing includes an inner wall and an outer wall, with a space defined between the inner wall and the outer wall and maintained in a vacuum state, thus insulating the cooking cavity from an outside of the inner casing. The steam oven further includes a steam generator to supply steam into the inner casing. Therefore, the steam oven accomplishes the desired insulating effect without using any insulating material while the space defined between the inner wall and the outer wall reduces conductive and convective heat transfer.