PURPOSE: To reduce the hardware of each trunk, by providing a trunk test circuit in common to each trunk.

CONSTITUTION: A test data DA is transmitted from a central controller CC via a line DL and an enable signal of a transmission register TRA is transmitted via a line EA and the DA is stored in the TRA. Similarly, test data DB, DC are stored to transmission registers TRB, TRC. The data DA, DB, DC are inputted respectively to input terminals Ai, Bi, Ci of a trunk C-TRK to be tested via a channel network TDNW from output terminals TA0, TB0, TC0, and when the C-TRK is normal, the data of DB+DC, DA+DC, DA+DB are outputted respectively from output terminals A0, B0, C0 and stored in reception registers RRA, RRB, RRC. The central controller CC reads in the data stored in the reception register and compares it with the test data transmitted to a transmission register, to discriminate the normality of the C-TRK.