PURPOSE: To eliminate the flicker of a liquid crystal screen so as not to give the feeling of incompatibility to users, and to prolong the service life of a battery by completely turning-off a backlight during the printing operation of a printer.

CONSTITUTION: According to a program stored in a memory part 5, a microcomputer control part 4 checks whether a backlight 3 is turned on or not, a backlight turning-on flag is set when the backlight is turned on, and the backlight 3 is turned off. Afterwards, the printing operation of the printer is started. During the printing operation of the printer in normal operations, since the user observes printing outputted at a printer part 6 and a liquid crystal display part 2 is not observed, there is no feeling of incompatibility even when the backlight 3 is turned off. Further, since the liquid crystal screen displays data although the backlight 3 is turned off, the display can be observed as needed. After the printing operation of the printer is completed, it is checked whether the backlight turning-on flag is set or not and when the flag is set, the backlight 3 is turned on again.