In this paper, we tried to evaluate the contact angle for a droplet on the solid surface with ruggedness based on elasticity theory and thermodynamics. In this theory, we assumed that the solid was homogeneous isotropic elastic body. Surface tension was given as function of strain in the imaginary layer which had thin finite thickness τ. By using this function and thermodynamics, cosine of the contact angle of a droplet was given as the function of volume fraction of solid in this imaginary layer. Ando, etc. had announced the experimental data for the porous silicon surface fabricated by anodic etching. For the purpose to confirm the validity of this theory, we compared these experimental data with theoretical calculated data. The theoretical calculated data showed comparatively good agreement with experimental data.