Though subject departmentalization in public libraries has often been discussed the method has not been employed in Japanese libraries except the partial adoption in the National Diet Library. The Chiba Prefectural Library, however, started to work by providing subject reading rooms since April, 1957. In future, large public libraries may try to employ tye above mentioned method taking Chiba's as the test case. Paying attention to factors related, the writer tried to introduce the planning and practicing in the Library.