The generation mechanism of the deposit which causes EGR valve stick in the Lean combustion was investigated. This deposit is easily generated by cooled environment at the time of transient state. The chemical analysis was conducted in order to check the chemical component which causes the EGR valve stick in this deposit. From the analysis result of emission gas and the deposit, the chemical factor which causes EGR valve stick was able to be found out. In order to check the credibility of this chemical factor, the reappearance examination using a diesel engine was actually done. The rightness of the factor probed in the chemical analysis by reappearance examination was checked. The factor which should be controlled in order to control the deposit carried out based on the above result is introduced.