  The primordial perturbation is widely accepted to be generated through the
vacuum fluctuation of the scalar field which drives inflation. It is, however,
not completely clear what is the natural vacuum in the inflationary universe
particularly in the presence of non-linear interactions. In this series of
papers, we will address this issue, focusing on the condition required for the
removal of the divergence from the infrared (IR) contribution to loop diagrams.
We show that requesting the gauge invariance in the local observable universe
guarantees the IR regularity of the loop corrections beginning with a simple
initial state. In our previous works, the IR regularity condition was discussed
using the slow roll expansion, which restricts the background evolution of the
inflationary universe. We will show more generally that requesting the gauge
invariance/the IR regularity leads to non-trivial constraints on the allowed
quantum states.
