Optical transmittance spectra of single-crystal YBa2Cu3O(y) thin films (y = 6.0, 6.4, 6.8) were measured in a wavenumber range of 1500-50000 cm-1 (0.2-6 eV) at room temperature. The electric field of incident light was parallel to the Cu-O planes. An absorption spectrum for each oxygen content was obtained from the transmittance spectra of two samples with different thicknesses. In YBa2Cu3O6.0, an energy gap at 1.5 eV and an absorption peak at 4.1 eV were observed. When the oxygen content increases, the absorption within the energy gap increases and the 4.1 eV absorption peak becomes smaller. We found that the dependence of the 4.1 eV peak intensity on the oxygen content correlates with that of the absorption within the 1.5 eV gap. Some discussion is given in connection with hole doping.