PURPOSE: To simplify the preparation of channel command words and data each of which consists of small bytes by providing a channel device with a channel command word storing part and transferring a part of the channel command words stored in the storing part directly to an I/O device.

CONSTITUTION: A flag bit for transferring the contents of a data address part 10 directly to an I/O device is prepared in a flag part 11 of a channel command word CCW, and when the CCW consists of small bytes, the bit is set to "1". The difference of outputs of the lower two bits between a data length part 12 and a data length counter 14 is found by a subtractor 18 and bytes of the data address part 10 are selected by a selector 19 and transferred to a data register 20. If the difference of the contents between the data length part 12 and the counter 14 is "0", "2" or "3", the selector 19 is constituted so that the 1st, 2nd or 3rd byte is transferred, respectively. After transferring said byte, the counter 14 counts down -1 by -1, and when the contents of the counter 14 becomes "0", an all detector 15 detects the value "0" and outputs an end signal 21 indicating the completion of data transfer in one CCW.