Design and operation of the vacuum system of the electron synchrotron at the Institute for Nuclear Study, University of Tokyo are described. The number of pumps to be installed around the donut ring of the electron synchrotron was determined by a quantitative estimation based on experimental data. Eight pumping units are installed around the donut ring. These units are all linked at the backside, and are backed by one fore-pump system consisting of an oil diffusion pump and a rotary pump. The role of the diffusion pump in the fore-pump system is discussed with results of pressure distribution measurement in the vacuum system under various conditions. Each pump unit used in the vacuum system of the synchrotron is operated without any cold trap.