The bending elasticity of monolayer membranes was studied by the use of the free-energy model, in which the energy of the surface dilations of head, chain, and intermediate surface of the molecule was assumed. The intermediate surface was inserted to reproduce the frustrated internal stresses within individual molecules. Under given curvatures, the area of the neutral surface and the position of it were determined so as to minimize the free energy. This means that there is no interfacial tension at all. The membranes in the bicontinuous phase of the microemulsions could be an example of our calculation. We have shown that there exists a neutral surface where the saddle bending modulus becomes zero when the intermediate part does not exist. This result is due to the fact that the free energy does not possess any term that is resistant to shear and it represents the liquid phase of membranes. Results for frustrated membranes with the intermediate surface were also reported.