PURPOSE: To test the software or hardware of an application device by an additional circuit, by connecting a testing device and the application device through an operating part, a control part and an interface circuit part for test information.

CONSTITUTION: A digital computer application device 1 is constituted of a CPU 11, a memory 12, input/output interfaces 13, 14, a control signal interface 17, and an address and data gate 18, and each of them is connected through a control bus 15, and an address and data but 16. On the other hand, a testing device 2 is constituted of a control signal gate 21, an address and data bus gate 22, a controlling circuit 23, an display device 24 and an operation switch 25, and the application device 1 and the testing device 2 are connected by a signal cable 3. In this state, by using the switch 25 and the display device 24 of the device 2, control of the CPU11, display and change of contents of the memory 12, and display and output of data of the input/output interfaces 13, 14 are executed.