PURPOSE: To automatically detect bad causes, by detecting the generation intervals and the terms of interlocking signals transmitted from a first unit to a second unit and comparing the intervals and the terms with the respective reference values of generation intervals and the term.

CONSTITUTION: The expected min. interval Tref and the expected min. term tref of the discharge direction signals are set in a combination scale 10 and a packer 12 which are an enbodiment of the present invention. When the discharge direction signal is received. the generation of the discharge direction signal and the extinction time are stored in a RAM 40 and the difference between a prior generation time and extinguish time is obtained to get the term T1, of the discharge direction signal and further, the difference of generated time or extinction time of the discharge direction signal generated before one from the generated time or extinction time of the objective discharge direction signal is obtained. And if the interval T1 of the discharge direction signal is less than interval Tref, as it is larger than the interval of discharge direction signal, a normal alarm of signal interval is given.