The lower critical field H-c1 is one of the important quantities for understandings of the fundamental behavior of type-II superconductors. We evaluated H-c1 for several oxide superconductors with various hole densities within the framework of the modified Kim-Anderson critical-state model. The correlation between H-c1 and hole density seems to be consistent with the results of muon spin relaxation (mu SR), i.e., H-c1 varies in proportion to the hole density in the CuO2 plane.