When a solid insulator is exposed to a high electric field in vacuum, electrons released from the triple junction at the cathode collide with the insulator and charge up its surface through an electron hopping process called the Secondary Emission Electron Avalanche (SEEA) mechanism. The process ultimately reaches an equilibrium state of charging.In a recent study, we have clarified that roughening the surface obstructs the charging since it provides a barrier against electrons in the hopping process. In this study, we apply this result to set the charging within a limited region on the surface of an insulator. That is, we have mechanically processed the surface of a cylindrical insulator into smooth and rough regions. By observing the charging with an electrostatic probe and an optical method, we show that electrification concentrates on the smooth region. In addition, we have succeeded in controlling the path of the flashover successive to the charging. The controlling method will help us to conduct further studies on the flashover mechanism in vacuum. © 2003, The Institute of Electrical Engineers of Japan. All rights reserved.