An image processing apparatus is provided which can execute a rotation process of an image when there is insufficient storage capacity. In a multi-function device to read images on two sides of a document by reversing a transfer direction of the document in an automatic document feeder, an estimated available capacity can be obtained based on a previously set condition, such as the resolution or gradation of an image scanner. Based on the estimated available capacity, it can be judged whether the rotation process can be performed. When the rotation process can not be performed, a process to reduce a set value of the condition is executed. When the set value of the condition is reduced, an attempt can be made to execute the process under the reduced set value.