Single-atomic-layered quantum wells built in wide-gap semiconductorsLnCuOCh(Ln=lanthanide,

$\mathrm{Ln}\mathrm{CuO}\mathrm{Ch}$ $(Ln=\mathrm{lanthanide},Ch=\mathrm{chalcogen})$ layered oxychalcogenides are wide-gap $p$-type semiconductors composed of alternately stacked ${(Ln}_{2}{\mathrm{O}}_{2}{)}^{2+}$ oxide layers and $({\mathrm{Cu}}_{2}{\mathrm{Ch}}_{2}{)}^{2\ensuremath{-}}$ chalcogenide layers. Energy band calculations revealed that $\mathrm{Cu}\ensuremath{-}Ch$ hybridized bands only spread in the $({\mathrm{Cu}}_{2}{\mathrm{Ch}}_{2}{)}^{2\ensuremath{-}}$ layers, which suggests that hole carriers in these bands are confined by the potential barriers formed by the ${(Ln}_{2}{\mathrm{O}}_{2}{)}^{2+}$ layers. Stepwise absorption spectra of a series of $\mathrm{Ln}\mathrm{CuO}\mathrm{Ch}$ experimentally verified that an exciton in the $({\mathrm{Cu}}_{2}{\mathrm{Ch}}_{2}{)}^{2\ensuremath{-}}$ layers shows a two-dimensional behavior. These theoretical and experimental results indicate that $\mathrm{Ln}\mathrm{CuO}\mathrm{Ch}$ has ``natural multiple quantum wells'' built into its layered structure.