  The KamLAND-Zen 800 experiment is searching for the neutrinoless double-beta
decay of $^{136}$Xe by using $^{136}$Xe-loaded liquid scintillator. The liquid
scintillator is enclosed inside a balloon made of thin, transparent,
low-radioactivity film that we call Inner Balloon (IB). The IB, apart from
guaranteeing the liquid containment, also allows to minimize the background
from cosmogenic muon-spallation products and $^{8}$B solar neutrinos. Indeed
these events could contribute to the total counts in the region of interest
around the Q-value of the double-beta decay of $^{136}$Xe. In this paper, we
present an overview of the IB and describe the various steps of its
commissioning minimizing the radioactive contaminations, from the material
selection, to the fabrication of the balloon and its installation inside the
KamLAND detector. Finally, we show the impact of the IB on the KamLAND
background as measured by the KamLAND detector itself.
