A memory control unit controls writing and reading of data to the slave device according to an instruction from the master device. A bus diagnosis line is directly connected from the bus signal control circuit to a bus signal receiving terminal of the slave device without passing through the address bus and the control signal line. A bus signal abnormality processing unit compares an output bus signal output from the bus signal control circuit to the address bus and the control signal line with a feedback bus signal fed back through the bus diagnosis line to determine the presence/absence of a difference. The memory control unit elongates a bus cycle period of a bus cycle of operation being executed when it is determined in the bus signal abnormality processing unit that the difference is present.