comparator 68 remains at a high level as shown in fig4 c and the and gate 69 provides an output signal as shown in fig4 d which is identical with the knocking detection signal of the form of a pulse train the integrator 3 integrates the output signal of the and gate as shown in fig4 e the phase delay controller 5 delays the phase of the reference ignition timing signal as shown in fig4 f according to the output voltage of the integrator 3 as shown in fig 4 e to provide a pulse signal as shown in fig4 g the transistor t r of fig1 is switched off by using the falling of the pulse signal to interrupt current supply to the ignition coil c i whereby the engine is ignited the ignition timing is indicated by a symbol 34 u03b4 34 in fig4 at the falling of the pulse as shown in fig4 g namely at the ignition timing the transistor 67 of fig3 is switched on to reset the charging voltage of the capacitor 63 to zero level as indicated at c in fig4 b fig5 shows the respective waveforms of signals generated by the component of the ignition timing control system of fig3 upon occurrence of a comparatively strong knocking when a knocking detection signal of the form of a pulse train having a plurality of pulses as shown in fig5 a is generated due to the occurrence of a strong knocking the charging voltage indicated at c rises to exceed the predetermined reference voltage v r as shown in fig5 b and.