A dually foldable host unit case for a computer includes a first plate member, an upper cover member, a lower cover, a front wall and a rear wall. The first plate member pivotally connects with the upper cover, the lower cover, the front wall and the rear wall respectively. The preceding parts are capable of being folded to allow the host unit case being reduced volume thereof while packaging and capable of being extended to allow the second lateral sides of the front and rear walls and the second lateral sides of the upper and lower covers are pivotally connected for setting up the host unit case conveniently and quickly.