An image forming apparatus includes a controller that operates as a memory device detection module that detects connection of the memory device to the port, a speculative process control module that, when the connection of the memory device is detected, reads sizes of the one or more image data items from the memory device, and causes the image processor to speculatively process the one or more image data items in a descending order of the sizes, and a job execution module that, when the job execution request is input, reads the speculatively-processed image data item from the storage device on a basis of the selection request, and outputs the speculatively-processed image data item in response to the job execution request.