[en] Present performance of KEK electrostatic separator (Mark I) and the results of mass separation test using this separator and described. Maximum voltage of 900 kV was obtained with the 3 m separator and of 800 kV with the 9 m separator across the electrode spacing of 10 cm. Mass separation test using the 9 m separator was attempted at the bubble chamber beam Kl, and mass separation between pion and proton was achieved at the momentum of 3.5 GeV/c and 2 GeV/c. Separation ratio of -- 3.6 between pion and proton was obtained. The relative kaon yields were enriched from -- 1/200 to -- 1/15 at the mass slit when the separator was tuned at the momentum of 3.5 GeV/c. (auth.)