A method of controlling data which is stored into a common storage in a multiprocessor system having a common storage which can be accessed from a plurality of computers is disclosed. Particularly, a method of controlling shared data using an extended storage as a common storage is disclosed. The copy of the data of a part of or all of a shared data set in an input/output device and the copy data control information to control the copied data are stored into the common extended storage. In the case of accessing the data in the common extended storage, the mutual exclusion among a plurality of computers and the consistency control of the data are realized by referring to the control information stored in the common extended storage.