PURPOSE: To select the request of the two request units equally, by providing the hold circuit where the state is inversed every time one of two request signals is received and the block signal generator blocking any reception of two request signals.

CONSTITUTION: The request signal 100 from the first request unit and the second request signal 200 are fed to the request receiving signal generating circuits 473, 474, to produce the request receiving signals 121 and 221. The AND circuit 410 detects the request signals 100 and 200 for simultaneous production, the output and the output of the request receiving signal generating circuit 473 are input to the AND circuit 420, and the output is input to the terminal J of the JK flip-flop. Further, the output of the request receiving signal generating circuit 474 is input. The output of the JK flip-flop 430 and the request signals 100, 200 are input to the block signal generating circuit 472, to block the reception of one request signal.