beginning of the overrun the wdt 63 does not output the r signal because of receiving the s signal irrespective of the fact it is capable of sensing the abnormality of the computer 62 by sensing lack of the p signal accordingly the microcomputer 62 is not reset and thus both the reverse lamp 72 and the reverse buzzer 73 are forced to stop their operation by a change of the operating signal from the overrunning microcomputer 62 however under this condition the s signal is inputted also to the auxiliary drive circuit 64 which simultaneously receives also the key switch on signal and the reverse switch on signal thus the qualifications of operation of the reverse lamp and buzzer are met which results in outputting an auxiliary operating signal from the auxiliary drive circuit 64 as a result even when the microcomputer 62 fails to output the operating signal due to an overrun the auxiliary operating signal is issued from the auxiliary drive circuit 64 to keep the transistor 66 on and thus the reverse lamp 72 and the reverse buzzer 73 are kept on as is described hereinabove when receiving the sleep signal from the microcomputer the key switch on signal and the reverse switch on signal the auxiliary drive circuit outputs the auxiliary operating signal replacing the operating signal from the microcomputer thereby enabling operation of the reverse lamp and reverse buzzer thus.