PURPOSE: To easily reproduce various pictures displayed at the display device of a man machine interface device later in a computer system connecting the man machine interface device to the main body of a computer.

CONSTITUTION: During a normal working period, image data transmitted from a main body 1 of the computer through a signal line to the display device are fetched from this signal line by a regenerative data preservation control part 7 and time sequentially preserved in a preservation memory 9. When reproducing the displayed picture such as generation of abnormality, the man machine interface device is disconnected from the main body 1 of the computer, and the preserved images in the past are reproduced on the screen of the display device by a data reproduction control part 11.