An image processing apparatus which is capable of solving a problem that printing cannot be performed due to a shortage of a storage area for image data after rendering of a secure print job, and reducing a wait time from authentication to start of printing. A first storage unit performs rendering on a received secure print job, and stores image data in a storage area. A first transition unit makes the secure print job transit to waiting-for-authentication state. A first suspending unit suspends rendering when a stored image data amount reaches a full capacity during rendering in the waiting-for-authentication state. A second transition unit makes the secure print job transit to a successfully-authenticated state. A second storage unit performs printing on the stored image data, erases the printed image data, starts rendering on a subsequent page, and stores image data obtained by converting the subsequent page.