To provide a multiple loop-back system, capable of more accurate conductivity evaluation of connection and route investigation without performing complicated processings.

A device passing order sequence number is defined in a free region of a loop-back cell. A device A11 which is a terminating point sends out the loop-back cell 101, for which the device passing order sequence number is '0' to an OAM(operation administration and maintenance) monitoring section. At the time of receiving the loop-back cell 101, the intermediate device B12, a device C13 and a device D14 of intermediate points respectively counts up the device passing order sequence number, loops back the loop-back cells 102, 103 and 104 to which the ID of the present device is added toward the device A and simultaneously sends out the loop-back cell for which the device passing order sequence number has been updated to a post stage. A device E35 of an opposite side terminating point resets the device passing order sequence number and loops back a loop-back cell 105, to which the ID of own device is added to the device A.