PURPOSE: To change color or brightness in display light according to various situations, and improve visibility in display content by arranging a changing means capable of changing the display light of a fluorescent display tube.

CONSTITUTION: Hardware and software of a printer controller 1 are set from a panel controller 5 of an operation panel 11, and its message is displayed on a fluorescent display tube 4 as a command setting message through a fluorescent display tube controller 3. The controller 1 decodes contents of the message displayed on the display tube 4, and gives priority as the message over those, and sends a brightness setting command to the controller 3 according to its priority, and changes brightness of the display tube 4 according to the contents of the message. Thereby, the brightness becomes maximum when an error message is displayed, and the brightness becomes minimum when a warming-up message is displayed.