In this study, Data Envelopment Analysis (DEA) method is introduced as a technique to measure the efficiency of public libraries, and is used to determine the relative efficiency of the city and ward libraries in Tokyo. Efficiency is defined as whether or not a library could reduce the inputs it uses equiproportionately and still produce the same output. Inputs are staffs, holdings, new books, number of libtaries and population. Output is circulation. We calculate the technical efficiency score (CCR efficiency score), pure technical efficiency score (BCC efficiency score) and scale efficiency score of each library. The subject libraries operate at 80% technical efficiency on the average, i.e., inputs could be reduce by 20% without sacrificing output if all libraries were as efficient as the benchmark efficient libraries identified by DEA.