PURPOSE: To improve the data processing speed, by making a magnetic controller execute a job to other magnetic tape devices other than a space file instruction during the inter-block gap period even if a space file instruction is executed.

CONSTITUTION: Since an inter-block gap IBG detection circuit 16 outputs "0" during the readout of a data block, "1" is outputted from an inverter 14 to form a busy signal and transmitted to a channel. The channel discriminate from this busy signal that a magnetic tape controller MTC is busy. When a magnetic head detects the IBG, the circuit 16 detects "1". The inverter 14 outputs "0" with this "1" and the busy signal level drops and a not-busy signal goes to "1". Thus, the channel can transmits other jobs to magnetic tape devices other than one executing a space file SPF instruction by observing the level-down of the busy signal.