PURPOSE: To improve the reliability of a system by detecting a bus collision.

CONSTITUTION: When a common control part 101 performs an access to an LA(i) 102 of a sequence number #i by indicating a line address (i) by a line address indicating means 107, response informing means (n)-(j) of LA(i)-LA(j) are in OFF states. When the LA(i) 102 detects the coincidence of the number by an address decode means 104, response informing means (i)-(l) of LA(i)-LA(l) 102 are turned to ON states, and a responding processing is operated for the request of the common control part 101. When the common control part 101 performs the access to the LA(i) 102 of the sequence number #i, the LA(j) 102 breaks down, and the response informing means (j) is turned ON, a bus competition error is communicated to the common control part 101 by a bus competition error detecting means 105 of the LA(i) 102, and a bus competition error informing means 108.