PURPOSE: To obtain a screen state at an optional point of time by holding the screen state as a sequence of pairs of corresponding screen operation instructions and reverse operation instructions and executing them by a mechanism which monitors screen operation.

CONSTITUTION: When a screen operation instruction monitor part 8 extracts an instruction for screen operation sent by an application program, a reverse operation instruction generation part 9 generates an instruction for putting the screen after the instruction is executed back to a current screen. This reverse operation instruction is generated corresponding to the instruction for above screen operation every time the instruction is generated, and the instruction is stored in an instruction stack part 10 in the generation order. When an operator who wants to reference past screen data indicates that by a predetermined method, a screen operation control part 12 takes the reverse operation instructions out of the instruction stack part 10 in order from an instruction which is stacked newly and passes them to an operating system(OS). The OS executes them. Consequently, the contents of the display screen can be put back to an optional time state and displayed.