PURPOSE: To attain the treatment of the null data item by dividing a composite key into sections having designated lengths for each data item and inserting separators to the points of divisions to give the value decided by the front/back situations to each separator.

CONSTITUTION: The data sent to a controller 1 via an input/output interface 12 is divided and compressed by an encoder 4 under the control of the controller 1. In addition the data on an index part 3 is detected via an encoder 5 and through a matching operation for front compression. A pair of a key and an address of the record on a data base 2 having said key are provided to the part 3 after division/compression and then the front compression. A desired record given from the base 2 is sent to the interface 12. A composite key is divided into sections having designated lengths for each data item for dividing compression. Then a separator having the value decided by the front/back situations is inserted to each pont of division.