To confirm normality or the like in jumper line construction or cable laying through a means easy to be handled without affecting an active line.

On a wiring frame 50, a monitoring device 1 is connected to the connecting terminal of a jumper line J2 of a terminal T4 connected with a cable C12 on the side of a slave device 51. Inside the monitoring device 1, a pulse selecting part 12 for extracting a pulse signal from an input signal and a transistor 13 for emitting a light emitting diode(LED) 14 by amplifying the output thereof are provided. When the pulse signal ordinarily outputted from the slave device 51 is returned from a cable C12, jumper line J1 to a cable C22 and the jumper line J2 by the HW-LOOP function of a host device 52, it is inputted to the monitoring device 1. On the monitoring device 1, the LED 14 is emitted by extracting and amplifying that pulse signal and a red lamp L1 is turned on so that normality or the like in the jumper line construction is confirmed.