PURPOSE: To improve transfer throughput by assembling paired-constitution transfer data and using a station unit, which receives a time slot, to analyze and reproduce the sense command of program control I/O commands.

CONSTITUTION: Loop control 2 is controlled by computer 1, and computer 1 and plural station units 4 which are interface of plural terminal equipments 5 are coupled in a loop through transfer line 3. Here, a paired-constitution transfer data format which is obtained by combining a sense command, which requests busy/ ready state information of terminal equipment 5, of program control I/O commands and another program control I/O command is adopted, and paired-constitution transfer data is assembled on the time slot received by each station unit 4, and the sense command of program control I/O commands analyzed and reproduced in station unit 4 which receives the time slot. As a result, transfer throughput can be improved.