An image output apparatus including a first storage unit and a second storage unit having a slower data access speed than the first storage unit, an input unit configured to input image data, a setting unit configured to set a threshold of remaining amount of the first storage unit, a detection unit configured to detect the remaining amount of the first storage unit in which the image data is stored, a determination unit configured to determine whether the remaining amount is smaller than the threshold, a control unit configured to control the storage units so that, when the determination unit determines the remaining amount is smaller than the threshold, image data which is smaller than the image data input by the input unit among the image data stored in the first storage unit is transferred to the second storage unit.