PURPOSE: To reduce the titled device, in size to add only a wiring at the increment of a terminal to be selected and to automate terminal connection by decoding processed data and selecting the I/O bus of a testing device.

CONSTITUTION: The testing device H is connected with a device G to be tested through an I/O terminal selecting circuit E and an auxiliary relay in the circuit E is driven in accordance with an output from an operation processor A outputting terminal information corresponding to a terminal selected on the device G side to select an I/O bus to be connected to the I/O terminal of the device G. The circuit E is provided with an automatic selecting part and a manual selecting part for testing terminals. The automatic selecting part connects the auxiliary relay through an OR circuit and distributes terminal information from the device A to drive the auxiliary relay. Consequently, the data from the device A are decoded and the I/O bus for the device is selected.