PURPOSE: To prevent the collision of data and to attain sure data transfer by dividing a storing means in the case of simultaneously and mutually sending data from two processes.

CONSTITUTION: A process l writes data (a) in a storage means 21 and a process 2 reads out the data (a) from the means 21 by waiting for the end of the writing (steps 1 to 3). Then the process 2 writes data (b) indicating the reading end of the data (a) in a storage means 22 and the process 1 reads out the data (b) to check the normal transfer of the data (b) (steps 4 to 6). Then the process 1 writes space data (c) in the means 21 for the preparation of writing of the succeeding data, the process 2 detects the data (c), writes space data (d) in the means 22 and the process l detects the data (d) (steps 7 to 10). operation erases the data of the means 21, 22 are preparing the succeeding communication. Data transfer from the process 2 to the process 1 is similarly executed by using storage means 23, 24.