PURPOSE: To make an engine ignitable surely at a low speed fixed position without making it excess timing advance even if a speed variation happens in time of low engine speed, by installing a speed variation preventing circuit.

CONSTITUTION: Input of a comparator 960 is connected to an operational condenser 108 and in time of idling speed, output of the comparator 960 comes to "zero". The output of this comparator 960 and that of a comparator 100 (a first fixed angle signal) both comes to the input of an AND circuit 970 so that in time of idling speed, the output of the comparator 100 will not come to a set signal of a flip-flop 300. Therefore, Q output of this flip-flop 300 is "zero" whereby the condenser 108 will not discharge out of a first fixed angle position θH. Accordingly, in a compression stroke, even if an engine speed drops in a moment, there is no discharge of the condenser 108 out of the angle position θH, never reaching to a reference voltage, ignition takes place at a low speed fixed angle θL without fail.