When we apply a voltage to a supported lipid bilayer self-spreading through a nanometer-scale gap (nanogap), the effects can be divided into two types. One is that there is no voltage-dependent change in the self-spreading behavior. Namely, the lipid bilayer passes through a nanogap without any stagnation. The other reveals that the self-spreading of a lipid bilayer can be controlled by an electric field modulation between nanogap electrodes. As a mechanism for these phenomena, we have proposed an electrostatic trapping model, in which the relationship between the thickness of an electric double layer and the nanogap spacing plays a crucial role. Here, to confirm the validity of this mechanism, we investigated the ionic concentration dependence of an electrolyte solution on the self-spreading behavior, which enabled us to tune the thickness of the electric double layer precisely. The result exhibited a certain threshold for controlling the self-spreading behavior. We also approximated the electric potential in the nanogap by using the Debye-Hückel equation. Our calculation result was in good agreement with the ionic concentration dependence experiments, suggesting the validity of our proposed mechanism. The results described in this work provide useful information regarding the realization of nanobio devices and the fundamental study of nanoelectronics.