PURPOSE: To stabilize ignition timing during low revolution speed by delaying the properties of advancing angle obtained from the computational result of an ignition timing computation circuit for a fixed time regardless of the number of revolution at least in a low revolution region.

CONSTITUTION: As the time chart C illustrates, the peak potential VT2, when charged and discharged, is constant below the specified number of revolution N2 regardless of the number of revolution. The potential VT2 either clips at constant- current voltage elements or clips by the source voltage by setting a large inclination of charging and discharge m2 and n2. Constant potential VT2 makes the inclination of charging and discharging m2 and n2 constant. The angle of discharging is smaller as the number of revolution is reduced. With this construction, an accurate and stable ignition timing can be obtained against revolution fluctuations at least fluctuations up to the specified number of revolution N2.