Adsorption on an n-type InAs surface often induces a gradual formation of a carrieraccumulation layer at the surface. By means of high-resolution photoelectron spectroscopy (PES), Betti et al. made a systematic observation of subbands in the accumulation layer in the formation process. Incorporating a highly nonparabolic (NP) dispersion of the conduction band into the local-density-functional (LDF) formalism, we examine the subband structure in the accumulation-layer formation process. Combining the LDF calculation with the PES experiment, we make an accurate evaluation of the accumulated-carrier density, the subband-edge energies, and the subband energy dispersion at each formation stage. Our theoretical calculation can reproduce the three observed subbands quantitatively. The subband dispersion, which deviates downward from that of the projected bulk conduction band with an increase in wave number, becomes significantly weaker in the formation process.