A copy button is pushed to output 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 a tone translator of the electronic blackboard. The image data on which image density control has been executed is written over the data previously stored in the USB memory.