PURPOSE: To supply an idle address at a high speed, by detecting independently the storage unit with which the reference is through and then storing the address of the storage unit to a buffer memory in the form of an idle address.

CONSTITUTION: The reference number set at a control memory 3 of a storage unit where an address is set at an address register 2 by a reference number replacement instruction is loaded to a counter 5 and then counted down in response to the reference. The storage unit with which the reference is over is detected by the counter 5 where the count contents is set zero independently. Then a timing control circuit 6 is controlled by the counter 5, and the contents of the memory 3 is written into a buffer 7 in the form of an idle address and in accordance with the writing indication given from the circuit 6. Thus an idle address can be supplied immediately and at a high speed in accordance with the reading of the buffer 7 and without retrieving the storage unit.