PURPOSE: To always monitor all memory cells inside DM without relying the setting contents of ACM.

CONSTITUTION: An idle time slot address holding circuit 6 selects the one optional time slot of the time slots where the control signal of an address control memory 4 indicates idle so as to hold the address. A timing generating circuit 7 generates a timing signal from address information of the above address holding circuit 6. A word-counter 8 executes counting with the one period of an address counter as one count and generates the address with the word number of a data memory 2 as full count. A selector 9 inputs the output address of the address control memory 4 or the word-counter 8 and selects the address of the word-counter by the timing of the timing generating circuit 7. The data memory 2 is read by the output address so as to monitor all the memory cells.