The purpose of this study is to reveal the ignition assistance mechanism in an alcohol fueled diesel engine. A motored two stroke cycle engine with a ceramic hot plug is motored, and one shot of spray is injected into the combustion chamber. Ignition lags are measured and splitted into physical and chemical lags by means of a statistical technique presented by S. Kumagai. High speed direct photographs are also taken. From the experimental results, it has been found that there are three kinds of hot surface temperature regions. In the low temperature region the mean value of ignition lags, the physical and chemical lags decrease exponentially with increasing the hot surface temperature. These decreasing behaviours are expressed by Arrhenius type equation. The activation energies of these three kinds of lags have the same value. In the higher temperature region the ignition lags are not affected by the hot surface temperature. Between these two regions a transient region is recognized. From the photographic observations it is also revealed that the hot surface does not initiate flame propagation but only gives heat to the fuel during its mixture formation and chemical reaction lead to spontaneous ignition.