PURPOSE: To improve the safety against the breakage of the screen data without deteriorating so much the screen editing operability by applying the fixed time limit to the continuous setting state of a screen data changing mode and switching automatically this changing mode to a read-only mode when a fixed time elapses after the screen data changing mode is set.

CONSTITUTION: A time 7 which applies the time limit t to the continuous setting state of a screen data changing mode is started with the mode switching key input applied from a keyboard 6 which switches a read-only mode to the screen data changing mode or an instruction given from a program. Then the timer 7 counts the time limit t and cancels the counting action of the t and restarts the time counting at 0 when a screen data changing command is received before the time-up state of the t for the input or the rewrite of the data. Therefore the read-only mode to the screen data changing mode or an instruction given from a program. Then the timer 7 counts the time limit t and cancels the counting action of the t and restarts the time counting at 0 when a screen data changing command is received before the time-up state of the t for the input or the rewrite of the data. Therefore the read-only mode or the screen data changing mode is set after the text file designated by a filing device 1 is loaded into a text buffer 3. Thus, the breakage of the screen data can be evaded.