The demand for clarifying the structure of non-stoichiometric mixture flames as well as that of stoichiometric mixture flames and the local burning mechanisms in turbulent flame has become urgent to establish the lean-burn technique in internal combustion engines from the viewpoint of environmental protection and energy conservation. In our previous work, a suggestion that the local burning velocity in turbulent flames was changed from the original laminar burning velocity was made based on experiments. However, the mechanism remains unclear. The purpose of this paper is to clarify this molecular behavior in more detail experimentally, and to discuss this mechanism. As a result, it was found that the Lewis number affected the local burning velocity in turbulent flames, and this effect was amplified by preferential diffusion.