Although　the　problem　of　library　size　is　one　of　the　classical　issues　in　the　field　of　libraryscience，　there　has　been　very　little　direct　discussion　on　it．　Then，　the　purposes　of　this　paper　areto　suggest　methodologies　for　defining　optimal　library　size　in　the　viewpoint　of　efficiency，　and　toverify　the　applicability　of　those．　　　In　this　study，　two　different　methodologies　were　conducted　using　the　statistical　data　ofJapanese　public　libraries　past　four　years　（1997－2000）．　Actual　analysis　was　performed　in　twolevels，　which　is　municipal－level　and　service　point－level．　lt　was　found　that　the　concept　oflibrary’s　efiiciency　isn’t　consistent　one．　Because　optimal　library　size　also　differ，　when　thecombination　of　variables　are　differed，　even　if　it　used　the　same　methodology．