[en] B factory is the accelerator for obtaining the knowledge on CP break by making the resonance state of T(4S) using the collision of electron-position, and investigating the decay process of B mesons arising from the T(4S). The measurement of the flight distance till B mesons decay becomes the central matter. It is necessary to be observed in the system of the moving T(4S). KEK, SLAC and Cornell University are going to promote concretely B factories in the world, and the constitution of that in KEK is explained. In this report, the case of utilizing the TRISTAN MR tunnel is described. The main parameters of the KEK B factory are shown. The design of the vacuum system is how to cope with the synchrotron radiation. At the design current value and the pressure of 10^-7Pa, the beam life in vacuum of more than 10 h can be assured. The exhaust system, the chamber material and the collision point are reported. It is necessary to forecast considerably accurately the detailed program of vacuum rise and the decrease of gas release rate due to synchrotron radiation. (K.I.)