PURPOSE: To prevent erroneous data readout by providing a transferring state indicator which instructs the data transferring state to the area of each reference unit at every reference unit of a shared extension storage.

CONSTITUTION: A transferring state indicator 3a which instructs the data transferring state to the area of each reference unit at every reference unit of a shared extension storage is provided in an extension storage device 3. When the data transfer from the main storage device 2 of a certain CPU 1 to a certain area of the extension storage device 3 using the indicator 3a is stopped by some abnormality, control is performed so that it can be informed to the CPU 1 referred to in accordance with the content of the indicator 3a when another CPU 1 refers to the said area of the device 3 that updating of the said area is stopped. Therefore, it can be recognized that the data in the extension storage of the said reference unit referred to are not correct.