To switch the font of a character string in a state in which the display operation of a display device such as a programmable display is maintained.

A table editor 32a of a plotting editor part 32 of a computer device 3 prepares a character string table including character strings. Fonts and control word addresses are set in the character string table. The character string table is downloaded to an FEPROM 122 of a programmable display 1 with picture data prepared by the plotting editor 32a. In the programmable display 1, when the font switching instruction of the character string is inputted from a user picture displayed on a display 14, a font switching part 20 refers to the control word address, and specifies the character string table of the control word address switched according to the font switching instruction, and switches the font of the displayed character string based on the character string table.