PURPOSE: To improve the reliability of a data processing system by obtaining the coincidence between the start points of effective periods of signals in case only a part of the signal is delivered together with another signal by a multiplex system and therefore avoiding that a wrong device is not transiently selected.

CONSTITUTION: A microprocessing unit MPU1 is actuated in an expanded multiplex mode, and upper 8 bits A8WA15 of an address are delivered onto a bus line 3a from the MPU1. At the same time, lower 8-bits A0WA7 of the address are delivered to a bus line 3b. Then 8-bit data D0WD7 are delivered to the bus line 3b. Addresses A8WA15 and A0WA7 are fetched simultaneously to latch circuits 4a and 4b synchronously with the fall of the address strobe signal AS. These addresses are supplied to a decoder circuit 5, and the chip selection signal CS and addresses A0WA15 are delivered to a high-speed signal processing LSI2.