In order to share out central resources fairly between low-speed and high-speed input/output ports 20-1, 20-2 of an input/output control unit, an activation time between a time point when an end status associated with the end of a transfer is sent and a time point when an activation request is received is measured and stored in a memory 48. When the high-speed port receives an activation request from a high-speed channel, the activation time in the memory measured with respect to the low-speed port is read out and during this activation time an input/output request for the low-speed port is preferentially accepted. To provide the activation time for the low-speed port the minimum time, average time and maximum time are obtained from the result of the measurement. Any of these three times can be selected as the activation time in order substantially to equalize the busy ratios of the high-speed and low-speed ports. Allocation can also be made in alternative ways, for instance by reference to the number of requests which meet with a busy signal.