particular the can system circuit 25 and the system power supply circuit 7 as shown in the figure the system power supply circuit 7 includes an ignition voltage hereinafter referred to as an ig voltage monitoring circuit 71 a voltage increasing circuit 72 a voltage decreasing circuit 73 and the 5v circuit 74 the ig voltage monitoring circuit 71 is connected to an ig input terminal 30 of the airbag ecu 20 and the ig input terminal 30 is connected in turn to the onboard battery 10 via the ignition switch 9 an input end of the voltage increasing circuit 72 is connected to the ig input terminal 30 a backup power supply 11 is connected between the voltage increasing circuit 72 and the voltage decreasing circuit 73 an ig voltage of for example 12v which is inputted into the airbag ecu 20 via the ig input terminal 30 is increased to 23v by the voltage increasing circuit 72 in order to change the backup power supply 11 in an efficient fashion thereafter the ig voltage is decreased to for example 7v by the voltage decreasing circuit 73 so as to be supplied to the 5v circuit 74 the 5v circuit 74 is such as to generate a system voltage for driving the microcomputer 3 and the like and includes a voltage fall detection circuit so as to output a reset signal to prevent a malfunction of the microcomputer 3 in case a voltage fall occurs the asic 21 also includes output circuits 5 for igniting airbags an overdrive detection circuit 31 for detecting an overdrive of the main microcomputer 3 and a serial communication circuit 32 for controlling a communication between the main microcomputer 3 and the asic 21 note that in the output.