To easily generate an operation screen by extracting data of a character string part from operation screen data and generating only the character string data corresponding to plural languages.

The display device 12 is connected to a specific external device 1 such as a programmable controller, and is so constituted as to perform display operation corresponding to data variation on the side of the external device 1 on the operation screen 11 of a display 16. In displaying method of the operation screen 11, character strings to be displayed on the operation 11 are extracted, a group of extracted character strings is saved as one character string file 23, and character string files 23 are prepared by different kind of languages needed to be displayed on the operation screen 11. On the other hand, prior to the display on the operation screen 11, a language to be displayed is determined, and corresponding character strings are taken out of the character file of the determined language and displayed on the operation screen 11.