The ignition lag in the spark ignition engine was determined by detecting the ion current through the spark plug itself, which was caused by the ions produced by the combustion immediately after the ignition spark. The influences of mixture ratio of fuel and air, electrode spacing and dilution with incombustible gas on the ignition lag of propane-air mixture in the closed vessel were made clear by this technique. In a two-stroke cycle gasoline engine, the ignition lag and the subsequent flame propagation and cycle-to-cycle variations in them were investigated. The results are as follows : (i) The cycle-to-cycle variations of ignition lag are hardly affected by the mixture ratio, the content of residual gas and by the engine speed. (ii) The cycle-to-cycle variations of the flame propagation and the maximum pressure in the cylinder are hardly affected by the cycle-to-cycle variation of ignition lag.