The surface-normal structure of a Cd layer formed on an Au(111) substrate using underpotential deposition (UPD) in a sulfuric acid solution has been investigated by specular X-ray reflectivity measurements. The measurements were carried out for the Cd UPD layers which were formed at the first and second UPD potential in electrolytic solutions of 1 mM CdSO4 with 10 mM and 100 mM H2SO4. In all cases, a Cd atomic layer is positioned on the Au(111) surface, which is similar to the reconstructed structure, and also a sulfuric acid anion layer is positioned on the Cd atomic layer. Contrary to results of the previous literature, the Cd coverage of the UPD layer depends not on the UPD potential but on the concentration of H2SO4. The coverage of the Cd atoms increases from approximately 0.50 to approximately 0.60 monolayer as the concentration of H2SO4 increases from 10 to 100 mM, and the coverage of the sulfuric acid anions is approximately half the Cd coverage.