KEK B-factory (KEKB) was a project to pursue a luminosity frontier of high energy physics after the energy frontier by TRISTAN. KEKB was designed making use of experiences during TRISTAN as a stepping-stone. From the experience of designing and operating the KEKB vacuum system, some topics of concern in those days are picked up and summarized. They are, synchrotron radiation, one-dimensional molecular flow, and the density measurement of electron cloud.