PURPOSE: To execute a test by an internal fold-back function and an external fold-back function for an external line, a disconnection test and a pseudo signal by providing plural line parts for containing the external line, and connecting annularly the external lines contained in its own line part and another line part, in each line part.

CONSTITUTION: A line part 11 outputs an internal line signal (g) inputted from a separating part 21 as a line signal (a) through the SW1 and the SW3, and in each line part 1n-12, the SW2 and the SW1 are operated, respectively and folded back to the outside, and when a line signal (b) is inputted to the line 11 by passing annularly through each line part 1n-12, the line part 11 outputs it as an internal line signal (m) through the SW4 and the SW2 which are not operated. Also, by operating the SW3 and the SW4, the internal line signal (g) inputted from the separating part 21 is folded back to the inside and outputted to a multiplexing part 20 as the internal line signal (m). Moreover, by operating the SW5, a signal (pseudo data) from a generator 31 is outputted as the internal line signal (m) through the SW5, the SW3, the SW4, and the SW2 which is not operated.