PURPOSE: To make inexpensive the constitution for a loop lock test, and to point out an abnormal part in a simple constitution by making the logical sum of the output of drivers in an arbitrary number into the input of a single receiver.

CONSTITUTION: In communicating equipment 4 having receivers 2-1 to 2-m in a smaller number than that of drivers 1-1 to 1-n, a loop back test connector 6 is mounted on a line connector 3, and the loop back test is executed. The loop back connector 6 has the constitution to logical-sum-connect the drivers 1-1 to 1-n in the arbitrary number as a set to one of the respective receivers 2-1 to 2-m by means of a diode 5, and a test process to detect whether or not a signal in an OFF-condition is inputted to the receivers 2-1 to 2-m belonging to the set when the signal which makes the all drivers 1-1 to 1-n in the arbitrary number into the OFF-condition is inputted, and the test process to detect whether or not the input of the signal in an ON-condition is inputted to the receivers 2-1 to 2-m belonging to the set by inputting the signal to successively make the drivers 1-1 to 1-n to the ON-condition are executed. From this, the loop back test including the all drivers and the all receivers can be continuously executed, and the simple abnormal part can be discriminated.