We have studied statistical mechanically the interplay between surface steps and adsorption with coverage 〓 being less than 1. As the surface model, we adapt the restricted solid-on-solid model coupled with Ising system (RSOS-I model). We calculate Andreev free energy, surface free energy per projected area, and the Andreev field dependence of the surface slope and the adsorption coverage by the transfer matrix method with product wave-function renormalization group algorithm. We have found that the thermal step bunching occurs when the lateral interaction among adsorbates exist, and show t...