A copy button is pushed to output an image data to an external storage medium, such as a USB memory. Then, a process for writing the image data in the USB memory is executed, and an image corresponding to the image data is printed. When the printed image is light, a density controlling button is pushed, and then, the copy button is pushed again. The image data is called up from a RAM, and a process for causing a density of the image data to be darker is executed, via image data on which image density control has been executed is written over the data previously stored in the USB memory.