PURPOSE: To describe a command by a previously determined single format by preparing a programless language interpreter for forming the contents of a command block so that the contents match with a format required for an opposite device.

CONSTITUTION: An I/O device 1 to be diagnosed receives an applied channel control word(CCW) and executes I/O operation. The programless language interpreter 2 forms a CCW required for the device 1. Namely the interpreter 2 includes a control block and forms the CCW of an old format or a new format so as to match the CCW with the sort of the device 1 based on the contents 3 of a CCW prepared by an operator. A test is executed based on the formed CCW. Thereby the operator can describe the contents of the CCW in accordance with the previously determined single format.