PURPOSE: To achieve a less man-hour and a higher reliability in the testing, by sending a signal from a line selection circuit in the order of the number on the core to compare the lighting condition of a display lamp at the input and output terminals thereof for reading the details of abnormality.

CONSTITUTION: As a signal is sent from a line selection circuit 11 in the order of number on the core, line display lamps L1WL10 attached at the input and output terminals of the core light when the core is normal. So, the lighting conditions at the input terminal and the output terminal are compared with a comparison circuit 13 to determine whether they are equal or not. when the lighting conditions are not equal, an abnormality signal is outputted and according to the abnormality signal, the details of the abnormality are interpreted with an interpreter 14 to be disconnection, short-circuiting or misconnection and the display lamps L11WL13 corresponding to the interpretation are lighted while a buzzer is driven to output a corresponding tone. Then, the testing of the next core is started by turning ON a continuity switch 16 for releasing the alarm of retaining interpreter 14 and upon the end of the testing of all cores, an end display lamp L14 comes ON.