PURPOSE: To check the serial-parallel converting function, etc. in a terminal device at a central device, by giving a function folded through an instruction from the central device to an interface section in the terminal device.

CONSTITUTION: In checking the data transmission state in a terminal device 20 from a central processor 1, a command information making active a signal (a) of the said terminal device from the central processor 1 is transmitted to a transmission line 11. When a serial-parallel conversion circuit 2 in the terminal device 2 receives it, the information is transmitted to a circuit 3, which makes the signal (a) active. Thus, the reception address of the information of a circuit 4 is switched from a terminal main body 6 (normally) to the circuit 3, and the information after the command information is returned to the central processor 1 via the circuits 3,4 and a circuit 2 as folding information. Thus, in the central processor 1, the transmission system including the serial-parallel conversion circuti 2 in the terminal device can be checked by comparing the transmission information with its folded information.