PURPOSE: To attain the effective use of a CPU and to improve the executing speed for treatment of a large quantity of data by using two storage means for communication of data carried out between a client and a server.

CONSTITUTION: The storage means 21 and 22 can be read and written by a server 60 and a client 50. The states of both means 21 and 22 are checked and the data are written in the writable one of both means 21 and 22, and a request is given to the server 60 via the data communication means 43 and 62 for processing the written data. If both means 21 and 22 are unwritable, a writing means 42 waits until the information showing that one of both means 21 and 22 becomes writable is received from the server 60 via both means 43 and 62. Meanwhile a reading means 61 takes the commands one by one out of the means 21 and 22 and then processed and outputs these commands after receiving the data processing requests of both means 21 and 22 from the client 50 via the means 43 and 62. In such a constitution, the wait processing operations can be extremely decreased and the executing speed of a window function is increased.