treatment of step 14 is taken at step 11 a figure 34 1 34 is added to the value of the sensor abnormality counter 4d at step 12 the target count value corresponding to the optimum ignition timing in the main program is read so that a target value for the ignition counter 4e is set at step 13 counting of the ignition counter 4e is initiated then other treatments such as control of over revolution and so on are executed and then the operation of the main program is again taken on the other hand the ignition counter 4e counts the number of pulses of the pulse signal from the crank angle sensor 1 when a counted value reaches a target value the operation is transferred to an ignition treatment loop as shown in fig3 b in which the engine is ignited step 20 and the sensor abnormality counter 4d is cleared step 21 then the operation is returned to the main program during the operation of the engine if an abnormal state occurs in the crank angle sensor 1 it does not generate any pulse signal then the count value of the ignition counter 4e does not reach the target value and the transfer to the ignition treatment.