detecting the number of revolutions the output signals from the pressure sensor 9 and the throttle opening sensor 10 are subjected to wave form shaping and noise removal in the second input interface circuit 102 and are thereafter successively a d converted by the a d converter 203 fuel injection is calculated by a valve opening period of the injector 7 and set in the timer 202 with or without correction during the time when the timer 202 is operative a voltage of a predetermined level is output from the output port 206 and subjected to voltage current conversion in the output interface circuit 103 to open the valve of the injector 7 the microcomputer 100 is operated by receiving a constant voltage from the power source circuit 104 to which the voltage of a battery 13 is input through a key switch 12 the operation of cpu 200 will be described by reference to fig4 at step s11 the number of revolutions of the engine n e is calculated from the measured value of the cycle of the ignition signal and then stored in ram 204 at step s12 the output signal from the pressure sensor 9 is a d converted by the a d converter 203 and stored in ram 204 as an a d conversion value of the intake pipe pressure pb ad hereinafter referred to as a pressure value because the pressure value pb ad contains a ripple component resulting from pulsation in the air suction the pressure value pb ad is subjected to a secondary low pass digital filter process.