PURPOSE: To attain ease of retrieval of cause to malfunction/inoperation by storing, reading and displaying a trouble data and an extracted data to a first-in first-out memory when the trouble data is detected.

CONSTITUTION: An address data and a control data on a multiplex transmission line are stored in a D flip-flop DF1, a fault so a multiplex transmission signal line is detected by a fault detection circuit TA to output and store a trouble data in response to the kind of fault. When at least one trouble data is outputted from a fault detection circuit TA, the trouble data, the address data stored in the D flip-flop memory DF1 and the control data are written in the FIFO memories M1, M2 and read by the operation of a read switch SW1 and displayed by light emitting diode groups LED1, LED2.