We developed a new cage lid made of stainless steel wire mesh and having a screen barrier for partitioning a small laboratory animal cage into compartments. To evaluate the effectiveness of the lid, we tested the transmissibility of Cilia-Associated Respiratory (CAR) bacillus from infected mice to uninfected sentinel mice, which were kept in separate compartments using this lid. Infection from the infected mice to the uninfected mice was confirmed by microbiological, serological, pathological, and molecular diagnostic examinations, as previously observed in an intra-cage contact route. The cage lid that we developed is very useful when uninfected mice are used in quarantine and contagion experiments to prevent fighting among the mice.