PURPOSE: To detect malfunctions by providing a step where the data are read out of a memory area indicated by an address and a step where the coincidence is decided between the produced data and the read-out data for the diagnosis of a memory.

CONSTITUTION: An address Ai and Di are produced in the order of storage from an address deciding part 11 and a data producing part 12 respectively after the data are written to a test area and then stored in registers 13a and 13b. A processing part 13 reads the data Dj out of the memory area of a RAM14 indicated by the address Ai and stores it to a register 13c. Then the part 13 checks the coincidence between both data Di and Dj. Then a malfunction is discriminated if no coincidence is obtained between both data. Thus an output is delivered to a CRT15 or a printer 16. While the part 13 requests both parts 11 and 12 for the next address Ai and data Di respectively when the coincidence is obtained between both data. Hereafter the RAM14 is diagnosed in the same way. Thus it is possible to detect accurately the malfunction due to the sudden change of a specific bit pattern or an address bus.